The Buddha is a title which means The Awakened One.
Gautama Buddha was one of a number of Buddhas who, according to tradition, appeared in history.
Gautama Buddha really did exist as a real person, and there is some archaeological evidence for this. Please see National Geographic's most recent article "Buddha's birthplace yields clues about his mysterious life".
In the Pali Canon, the word Brahma refers to a class of high level gods, who are not immortals. In fact, no person exists forever in Buddhism. Also, the word brahman in the Pali Canon refers to what the Hindu scriptures call brahmana or brahmin, the priestly class of people.
Nowhere does the term Brahman, as referring to God or the Ultimate Reality or Ultimate Truth, exist in the Pali Canon.
After he passed away, there is no way for anyone to contact Gautama Buddha anywhere by prayers or anything like that. By attaining enlightenment, you don't become one with the Buddha and you certainly don't become spiritually connected to him. That doesn't fit in with the notion of Brahman in Hinduism.
"Tathāgata" is formed by "tathā" + "gata" meaning "one who has thus gone". It has nothing to do with Hinduism's "tat twam asi" (That thou art i.e. you are that Ultimate Reality).
The idea of Brahman in Hinduism is a personification of the Ultimate Truth or Ultimate Reality. The Buddha rejected this kind of view as eternalism (sassatavada in Pali or sāśvata-dṛṣṭi in Sanskrit) in many suttas.
Some people say Nibbana is Brahman, but Nibbana is simply what is experienced by the mind which is free from defilements and fetters. It is not an Ultimate Reality i.e. it's not an eternal state of existence.
If we venture into Mahayana Buddhism, some Hindus may say that the Buddha Nature is Brahman. But the Buddha Nature is simply the potential to become a Buddha. It's not any kind of Ultimate Reality or Ultimate Truth.
In Mahayana Buddhism, some may say that the Buddha exists eternally in his Dharma Body. This comes from a saying of the Buddha in the Pali suttas to the effect of "when you see me, you see the Dharma. When you see the Dharma, you see me". This means that there is nothing special about the Buddha's form or personality to idolize. What is special about the Buddha is the Dharma - his teachings.
There is no such thing as Cosmic Man or Cosmic Consciousness in Buddhism. Consciousness is impermanent and dependent on sensory media. It doesn't exist independently from the six senses, looking through multiple living beings simultaneously.