In a small bowl, mix together all dry cake ingredients with a fork. Add in wet ingredients, stirring until combined. Heavily grease three large mugs* and evenly distribute batter into mugs (~1/2 cup of batter each).
Microwave each mug by itself for 1 minute on high. Remove from microwave and shake slightly to loosen cake from mug. Flip mug over onto parchment paper-lined plate or board. The cake should drop right out. Repeat with remaining mugs, then place into freezer for 5 minutes, or until the cakes cool to room temperature.
While the cakes are cooling, make the frosting. Combine all ingredients in a standing mixer and beat on high until light and fluffy, about 1 minute. Remove cakes from freezer and frost the top of each layer before stacking. If using, shave dark chocolate over the top of the cake using a sharp knife.
Notes
*don’t have 3 uniform mugs? Use one, cleaning thoroughly between cooking each layer!