Pre-heat the oven to 350° F.
Cream (or mix) the butter and sugar together.
Then add the eggs and vanilla, mixing well.
Whisk or sift together the flour, baking powder, cocoa powder, and salt.
Mix the dry ingredients into the creamed mixture alternately with the milk, beginning and ending with the dry ingredients. If the batter is too thin, you may need to add just a bit more flour.
Spray the whoopie pie pan with cooking spray. Drop 1-2 teaspoonfuls of cookie dough into each heart and smooth out.
Bake at 350° F for about 10 to 12 minutes.
Remove the cookies from the cookie sheet and place on a wire rack to cool.