Preheat oven to 375F.  Line two baking sheets with parchment paper.

Sift together flour, cocoa powder, baking soda, and salt into a medium-sized bowl.

In a large bowl with an electric mixer, beat together the butter, shortening, and brown sugar on low speed until combined.  Increase the speed to medium and beat until light and smooth.  Add the egg and vanilla and continue beating for another minute.

Add half of the flour mixture and half of the milk to the batter and beat on low speed until combined.  Add remaining flour and milk and beat until smooth.

  Using a tablespoon, drop the batter onto baking sheets at least 2 inches apart.  Bake each sheet individually for about 10 minutes, or until the cakes spring back when pressed.  Remove the sheet from the oven and let cool.

Spoon frosting into a pastry bag or small plastic baggie with one corner cut off.  Squeeze out approximately 2 tablespoons of frosting onto one cake, and top with a second cake.  Repeat until all cakes are used.

Choose either:

Classic Marshmallow Frosting

Chocolate Buttercream Frosting