1. Preheat oven to 350°F (176°C). Prepare a cupcake pan with cupcake liners.
2. In a large mixing bowl, beat the butter and sugar together on medium speed until light in color and fluffy, 3-4 minutes.
3. Add the sour cream and butter and vanilla extracts and mix until combined.
4. Add egg whites in two batches, mixing until incorporated after each addition. Scrape down the sides of the bowl as needed to make sure everything is combined.
5. Combine flour, baking powder and salt in a medium bowl. Combine the milk and water in a small measuring cup.
6. Add half of the flour mixture to the batter and mix until combined.
7. Add the milk mixture to the batter and mix until combined.
8. Add the remaining flour mixture and mix until smooth. Scrape down the sides of the bowl as needed to ensure everything is being combined.
9. Stir in the sprinkles.
10. Fill cupcake liners about halfway. Bake for 15-17 minutes, or until a toothpick inserted comes out with a few crumbs
11. Remove cupcakes from oven and allow to cool for 2-3 minutes, then remove to cooling rack to finish cooling.
13. While cupcakes cool, make the frosting. Combine egg whites, sugar, and cream of tartar in a metal, heatproof mixer bowl. Place bowl over a pot with simmering water (You could also use a double boiler)
14. Whisk constantly until sugar is dissolved and whites are warm to the touch, 3 to 4 minutes. Remove from heat.
15. Using the whisk attachment, beat on low speed, gradually increasing to high, until stiff, glossy peaks form, 5 to 7 minutes.
16. Frost the cupcakes (I used the large round Ateco 808 tip) and top with sprinkles. Add a little marshmallow popcorn treat to the top.