Whoopie Pies

Dough

• 8 ½ oz whole milk
• 1 ½ tbsp lemon juice
• 8 oz light brown sugar
• 4 oz unsalted butter, in pieces, room temperature
• 1 large egg
• 1 tsp vanilla extract
• 9 oz all-purpose flour
• 2 oz Dutch-processed cocoa powder
• 1 tsp espresso powder
• 1 tsp baking powder
• ½ tsp baking soda
• ¾ tsp salt

Marshmallow Creme Filling

• 10 oz unsalted butter, in pieces, room temperature
• 16 oz confectioners sugar
• 1 tsp vanilla extract
• ⅛ tsp salt
• 7 oz marshmallow creme