Preheat oven to 350°F.
Line baking sheets with parchment paper.
Mix flour, cocoa, baking soda, and salt.
Beat butter and sugar until fluffy.
Add egg and vanilla, beat until smooth.
Add dry mix and buttermilk alternately.
Drop spoonfuls onto sheet, space evenly.
Bake 10 minutes, until set.
Cool cookies on rack completely.
Beat butter and powdered sugar until fluffy.
Add marshmallow fluff, beat until creamy.
Tint filling with orange coloring.
Pipe filling onto half the cookies.
Top with another cookie to sandwich.
Decorate with Halloween sprinkles if desired.