Preheat oven to 350°F. Line baking sheets with parchment paper.

In a large bowl, combine flour, cocoa powder, baking powder, baking soda, salt, and sugar.

Add milk, vegetable oil, and vanilla to the dry ingredients and mix until smooth.

Drop spoonfuls of batter onto prepared baking sheets, spacing them 2 inches apart.

Bake for 10-12 minutes, until cookies are puffed and spring back when touched.

Let the cookies cool completely before filling.

Prepare the filling: Beat softened butter and confectioners' sugar until creamy. Add milk and vanilla, and mix until smooth.

Spread or pipe the filling onto the flat side of one cookie, then top with another cookie to make a sandwich.