Preheat the oven to 400°F. Line a 12-cup muffin tin with paper liners; set aside.
In the bowl of a stand mixer fitted with the paddle attachment or with a handheld electric mixer, beat the butter and sugar on medium speed until light and fluffy.
Beat in the sour cream followed by the egg whites and vanilla extract until well incorporated.
In a separate medium bowl, whisk together the flour, baking soda, and salt. Add to the butter mixture and beat until smooth and satiny. Stop to scrape down the sides of the bowl as needed.
Using a silicone spatula, fold in the M&M'S until evenly distributed throughout the batter.
Divide the batter evenly among the muffin cups and bake for 5 minutes. Reduce the oven temperature to 350°F and bake for an additional 10-12 minutes or until a toothpick inserted into the center comes out with only a few moist crumbs.
Cool the cupcakes in the pan for 5 minutes then transfer to a wire rack to cool completely.
Pipe a swirl of vanilla frosting over the top and garnish with Rice Krispies.
Make ahead tip
The cupcakes can be made 1 day in advance before assembling. Cover and refrigerate each overnight. Frost the cupcakes the next day.
Frosted cupcakes can be stored in an airtight container in the refrigerator for up to 3 days.
Both frosted and unfrosted cupcakes can be stored in an airtight container and frozen for up to 3 months. Thaw in the refrigerator overnight.
Nutritional values here on my recipes should be used as a general guide only. Since different brands of ingredients have different nutritional information, the values shown are just an estimate.