fresh
creamy
rancid
melted
Brush the pastry with a little melted butter.

softened
clarified
salted, unsalted
brandy (esp. BrE), garlic, herb
apple (AmE)
almond, cocoa
peanut

knob (BrE), pat, slab, stick (AmE)
He put a large knob of butter on the potatoes.
You can make frosting out of half a stick of butter and two cups of powdered sugar.

put on, spread (sth with)
Put some butter on the crackers, please.
He spread butter on the roll.

heat, melt, soften
add, beat, beat in, cream, mix, rub, rub in
Cream the butter and icing sugar together until light and fluffy.
Rub the butter into the flour.

churn, make

spread
This butter doesn't spread very well.

melt
The butter melted in the heat.

sauce
dish, knife

in butter
spinach sautéed in butter

bread and butter

generously, thickly (esp. BrE)
lightly
lightly buttered toast

Butter is used with these nouns as the object:
bread, toast