1. Preheat the oven to 325 degrees, line a cupcake pan with paper liners.
2. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. In a separate small bowl, stir together the milk and vanilla.
3. In the bowl of a stand mixer, beat the butter and sugar on medium-high speed until light and fluffy, about 2 minutes. Reduce the speed to medium-low, slowly add the egg and egg whites one at a time, and beat until creamy, About 1 minute.
4. Slowly add half the flour mixture, then the milk mixture, then the remaining flour mixture, beating until just blended after each addition.
5. Fill the liners about 2/3 full and bake until the tops are just dry to the touch and a toothpick inserted into the center comes out clean, 18-20 minutes. (20-22 minutes for a cake) Transfer the pan to a wire rack and cool completely before frosting with milk chocolate cream cheese frosting.
6. Frosting: In the bowl of a stand mixer with the paddle attachment, beat the cream cheese, butter and salt on medium-high speed until light and fluffy, about 2-3 minutes.
7. Slowly add the melted chocolate and beat for 1 minute more. Reduce the speed to low and gradually add the powdered sugar until just incorporated, being careful not to beat too much air into the frosting.
8. Add the milk and vanilla and beat until just combined. If necessary, add more milk 1 tablespoon at a time until the frosting has a spreadable consistency but is still thick and fudgy.

Original recipe adapted from Sprinkles Cookbook