Frankenstein Chocolate Whoopie Pies

• 2 cupsall-purpose flour
• ¾ cupunsweetened cocoa powder
• 1 tspbaking powder
• ½ tspbaking soda
• ½ tsptable salt
• ¾ cupunsalted butter — room temperature, 1½ sticks
• ¾ cuplight brown sugar
• ¾ cupgranulated sugar
• 2large eggs
• 2 tspvanilla extract
• ¾ cupbuttermilk

For the Filling

• ½ cupunsalted butter — room temperature, 1 stick
• 2 cupsconfectioners' sugar
• 2 tbspmilk
• 1 tspvanilla extract

For Frankenstein decorations

• 40mini marshmallows
• 40candy eyes
• ¼ cupwhite chocolate — melted
• —green food coloring