It is my basic understanding that certain actions such as killing one's parents or harming the Buddha has immediate karmic reactions. Are there actions that have immediate "wholesome" outcomes or ones that allow you to start with a clean slate?
There is a very famous simile in the Lonaphala Sutta: The Salt Crystal. An excerpt:
"There is the case where a trifling evil deed done by a certain individual takes him to hell. There is the case where the very same sort of trifling deed done by another individual is experienced in the here & now, and for the most part barely appears for a moment.
The Buddha gives a simile of dropping a salt crystal into a cup and asking if the water was fit to drink and, of course, the answer is no. It would be too salty due to the small amount of water in the cup. The Buddha then asks if a salt crystal were to be dropped into the Ganges River if it would be fit to drink and, or course, the answer is yes because of the large amount of water in the river.
So the same action does not necessarily lead to the same effect nor to the same length of time for the ripening of the effect. The Buddha then gave some very helpful details of what type of people may experience lessor effects of their bad karma.
Now, a trifling evil act done by what sort of individual takes him to hell? There is the case where a certain individual is undeveloped in the body, 2 undeveloped in virtue, undeveloped in mind [i.e., painful feelings can invade the mind and stay there], undeveloped in discernment: restricted, small-hearted, dwelling with suffering. A trifling evil act done by this sort of individual takes him to hell.
'Now, a trifling evil act done by what sort of individual is experienced in the here & now, and for the most part barely appears for a moment? There is the case where a certain individual is developed in the body, developed in virtue, developed in mind [i.e., painful feelings cannot invade the mind and stay there], developed in discernment: unrestricted, large-hearted, dwelling with the immeasurable. A trifling evil act done by this sort of individual is experienced in the here & now, and for the most part barely appears for a moment.
So while there may not be an immediate reset button to avoid the effects of past karmic actions, there is the idea of performing wholesome actions now and building merit to dilute "the salt" of past bad deeds and to become the type of person who experiences lessor effects of bad karma. Some people concentrate on building merit by giving offerings (dana), by practicing virtue (sila), and by practicing meditation (bhavana) in this effort.
It's because your slate is dirty, that you have come into being in this very same body and are subjected to suffering.
By attaining Nirvana in this very same body, your slate is not cleaned - you just become unaffected by your dirty slate due to attainment of Nirvana.
When you become unaffected by your dirty slate, your dirty slate will have no effect, thus you will enjoy a happy life no matter what happens to you in this very same life.
When you become unaffected by your dirty slate, your dirty slate will have no effect, thus you will enjoy a happy death in this very same life no matter how you die.
When you become unaffected by your dirty slate, your dirty slate will have no effect, thus it will not perpetuate future births after your death and you will be what you truly are.
When you are what you truly are, the rest is extinct, even your slate.
As you can see, you don't need to clean your slate to enjoy a happy life. You just need to find a way leading to Nirvana, thus making your mind unaffected by your slate.
The dirtier your slate is, the harder it will be for you to find a way leading to Nirvana. The dirt will cloud your mind and you won't be able to realize the truth leading to Nirvana.
No matter how dirty a slate is, the more a being strives to live life as Buddha taught his followers, the more the slate will become cleaner.
Thus, following the teachings of the Buddha is the way to clean ones slate enough in order to attain Nirvana.
Kamma is actually Fourfold. Take a look at the picture i have attached. It shows the four aspects of kamma, i.e. Function, Ripening, Time of Ripening and Place of Ripening.
Click on photos for full size
It will be too lengthy to treat all four aspects here so i would recommend the reading of the chapter about kamma. It can be found in the book "Comprehensive Manual of Abhidhamma" by Ven. Bhikkhu Bodhi, p. 200-220.
I no not know how to answer the "resetting-aspect" of the question so i will instead address the 3. aspect of kamma, i.e. the Time of Ripening of kamma and in here the "Immediately effective kamma".
First a definition of this type of kamma:
"Immediately effective (diμμhadhammavedan2ya) kamma is kamma which, if it is to ripen, must yield its results in the same existence in which it is performed; otherwise, if it does not meet the opportunity to ripen in the same existence, it becomes defunct. According to the Abhidhamma, of the seven javanas in a javana process, the first javana moment, being the weakest of all, generates immediately effective kamma"
Here are 2 examples of "Immediately Effective Kamma", p. 280-281:
"A husband and his wife possessed only one upper garment to wear when they went out-of-doors. One day the husband heard the Dhamma from the Buddha and was so pleased with the Doctrine that he wished to offer his only upper garment, but his innate greed would not permit him to do so. He com- batted with his mind and, ultimately overcoming his greed, offered the garment to the Buddha and exclaimed “I have won, I have won.” The king was delighted to hear his story and in appreciation of his generosity presented him thirty-two robes. The devout husband kept one for himself and another for his wife and offered the rest to the Buddha."
"A hunter who went hunting to the forest, followed by his dogs, met by the wayside a Bhikkhu who was proceed- ing on his alms round. As the hunter could not procure any game he thought it was due to the unfortunate meeting of the Bhikkhu. While returning home he met the same Bhikkhu and was deeply enraged at this second encounter. In spite of the entreaties of the innocent Bhikkhu the hunter set the dogs on him. Finding no escape therefrom, the Bhikkhu climbed a tree. The wicked hunter ran up to the tree, and pierced the soles of the Bhikkhu’s feet with the point of an arrow. The pain was so excruciating that the robe the Bhikkhu was wear- ing, fell upon the hunter completely covering him. The dogs, thinking that the Bhikkhu had fallen from the tree, devoured their own master."
Important note: The "Javana-process" of a thought-process is being mentioned. If you have questions about that i will try to explain it but if i do it here the post will be too lengthy and complex so maybe you could ask a new question about that or we could have a chat about it. Let me know if it becomes actuality.
A complete reset? I'm afraid not, unless you can save the entire world. I might recommend the book "Instant Karma: 8,879 Ways to Give Yourself and Others Good Fortune Right Now" with instruction on starting the process with practical suggestions ranging from quick and easy to very involved. It's simple but not easy. Unlike some religions in which you simply swear an oath, good karma demands effort and hard work. That is what will change your world and your life.