Preheat oven to 400°F. Line a muffin pan with cupcake liners.

In a mixing bowl, combine butter and sugar. Beat using an electric mixer for 1-2 minutes until fluffy. Add in egg whites, one at a time, lightly mixing with a spatula after each one. Add in sour cream, milk, and vanilla extract. Beat lightly.  

In a separate mixing bowl combine flour, baking soda, and salt. Gradually add flour mixture to wet ingredients a few spoonfuls at a time.

Add in mini M&Ms, mix until just incorporated. Fill cupcake liners 3/4 full.

Bake for 5 minutes at 400° F. Keeping cupcakes in the oven, turn heat down to 350°F and bake for 8-9 minutes, or until a few moist crumbs remain on a toothpick when inserted into center.

Allow cupcakes to cool completely before adding frosting.

To prepare icing, combine butter and vanilla extract. Beat using an electric mixer until fluffy.

Add powdered sugar, about 1 cup at a time, mixing by hand. Continue adding powdered sugar until frosting no longer looks wet and is sweetened to your liking. Next use an electric mixer to mix frosting until powdered sugar is fully incorporated. If frosting becomes too thick, add a dash of milk or heavy cream.

Place butter cream in a piping bag fit with your desired tip. Pipe frosting onto cupcakes and top with mini M&Ms, if desired.

Store in an airtight container for up to 3 days. If storing cupcakes prior to serving, add M&Ms to frosting right before serving (M&Ms can lose their color from sitting on frosting).