Yes, BUTTERNUT is a Scrabble word!

Letter values in Scrabble US: B 3, U 1, T 1, T 1, E 1, R 1, N 1, U 1, T 1.

Is it a valid word?

BUTTERNUT is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

North American walnut tree having light-brown wood and edible nuts; source of a light-brown dye
oily egg-shaped nut of an American tree of the walnut family

Also: butternut tree, white walnut, juglans cinerea.