Chocolate Whoopie Pies

Name: Chocolate Whoopie Pies
Ingredients: 1 1/4 tsps baking soda; 1/2 lb butter (2 sticks), at room temperature; 1 cup buttermilk; 1/2 cup cocoa; 1 large egg; 3 large egg whites; 2 cups all-purpose flour; 1 cup light brown sugar; 1/4 tsp sea salt; 3/4 cup sugar; 3/4 tsp vanilla; 1 tsp vanilla extract