Preheat and prep. Heat the oven to 350°F (175°C).
Line a 12-cup muffin tin with cupcake liners. If you have two tins, you’ll get 18 to 24 cupcakes total, depending on size.
Mix dry ingredients. In a medium bowl, whisk 2 cups all-purpose flour, 2 teaspoons baking powder, and 1/2 teaspoon fine salt. Set aside.
Cream butter, oil, and sugar. In a large bowl, beat 1/2 cup unsalted butter (room temperature), 1/4 cup neutral oil, and 1 cup granulated sugar on medium-high until pale and fluffy, about 2 minutes.
Add flavorings and egg whites. Beat in 1 teaspoon vanilla extract and 1/4 teaspoon almond extract.
Add 4 egg whites, one at a time, mixing on medium until smooth. Scrape the bowl as needed.
Alternate dry and milk. With the mixer on low, add half the dry mix, then 3/4 cup whole milk, then the remaining dry mix. Mix just until combined.
Do not overmix.
Color the batter (optional). If making multicolored cupcakes, divide batter into 3–4 bowls. Tint each with a small amount of gel food coloring. Stir gently to avoid deflating the batter.
Add sprinkles (optional). Fold 1/3 cup rainbow jimmies into the base batter or into individual colors.
Avoid nonpareils; they tend to bleed.
Fill the liners. For solid-color cupcakes, fill each liner about 2/3 full. For rainbow swirls, add small spoonfuls of different colors, layering until 2/3 full. Tap the pan lightly to settle the batter.
Bake. Bake 16–20 minutes, rotating the pan halfway through.
Cupcakes are done when the tops spring back or a toothpick comes out with a few moist crumbs.
Cool completely. Let cupcakes cool in the pan for 5 minutes, then transfer to a rack. Cool completely before frosting.
Make the frosting. Beat 1 cup unsalted butter (room temperature) on medium-high until creamy, about 2 minutes. Add 3 1/2–4 cups confectioners’ sugar in stages, 1 teaspoon vanilla, a pinch of salt, and 2–4 tablespoons heavy cream or milk, beating until fluffy and smooth.
Color the frosting. Divide frosting into bowls and tint with gel colors of your choice.
For a swirl effect, add stripes of different colors to a piping bag fitted with a large star tip.
Decorate. Pipe generous swirls onto cooled cupcakes. Top with extra sprinkles. For a neat finish, hold the bag straight up, apply even pressure, and release just before lifting.
Serve. Let the frosting set for 10–15 minutes.
Enjoy at room temperature for the best texture and flavor.