Preheat oven and liners:

Heat oven to 350°F (175°C). Place paper liners in a standard cupcake pan.

Whisk dry ingredients:

In a medium bowl, whisk flour, cornstarch, baking powder, and salt together.

Cream butter and sugar:

Using a hand mixer or stand mixer with paddle, beat butter and sugar on medium speed until light and fluffy, about 3 minutes. Add egg whites, vanilla, and almond extract; mix until blended.

Beat in sour cream:

Scrape bowl sides, then beat in sour cream or yogurt on medium until smooth.

Alternate dry and milk:

Add dry ingredients in two additions, alternating with milk, beating on high about 10 seconds after each. Take bowl off mixer; fold in sprinkles with a spatula.

Bake cupcakes:

Fill liners ¾ full. Bake 15-18 minutes until a toothpick inserted in center emerges clean. Cool fully on a wire rack.

Beat butter for frosting:

In a large bowl with hand mixer or stand mixer paddle, beat butter on medium until creamy, roughly 2 minutes.

Make buttercream frosting:

With mixer on low, slowly add powdered sugar, vanilla, almond extract, and milk or cream. Raise speed to medium and beat until smooth, about 3 minutes. Add salt to taste. Fold in sprinkles with a spatula. If too stiff, add more milk; if too thin, add more powdered sugar. Tint with gel coloring if using.

Pipe and decorate cupcakes:

Load a piping bag (or zip-top bag with a snipped corner) with frosting and decorate cupcakes. For a decorative finish, use a Wilton 1M tip. Garnish with extra sprinkles if desired. Keep in an airtight container at room temperature for up to 3 days.