Instructions

• Brown the butter: place 1 stick unsalted butter in a small saucepan over medium-high heat. Stir almost constantly for about 3 minutes, until the butter turns amber/golden brown and smells slightly nutty. Remove from heat and immediately pour the browned butter into the large mixing bowl you will use for the batter.
• Chill then bring the browned butter to a soft, room-temperature state: place the bowl of browned butter in the refrigerator or freezer just until the butter firms up (this makes it easier to work with). Then let the butter sit at room temperature until it is solid but soft—when you press a finger into the top it should make a slight indentation.
• Preheat the oven to 350°F. Line a 12-mold cupcake tin with paper liners and lightly spray the liners with non-stick spray; set the prepared pan aside.
• Combine the dry ingredients: in a large bowl whisk together 2 cups all-purpose flour (do NOT pack the flour; spoon and level), 1 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/2 teaspoon salt. Add 1/3 cup rainbow sprinkles to the dry mixture and toss to coat (this helps prevent the sprinkles from sinking).
• Combine the wet dairy: in a small bowl stir together 1/2 cup whole milk and 1/2 cup full-fat sour cream until smooth; set aside.
• Cream the butter and sugar: in the bowl with the softened browned butter, using a handheld electric mixer or stand mixer fitted with the paddle attachment, beat 1 cup granulated sugar and the browned butter on medium-high speed until light and fluffy, about 4 minutes.
• Add the eggs: beat in the 2 large eggs and then the additional large egg yolk, one at a time, beating well after each addition.
• Combine batter: reduce mixer speed to low. Add the flour mixture to the butter mixture in two additions, alternating with the milk/sour cream mixture—begin and end with the flour mixture (flour, dairy, flour). Mix only until each addition is incorporated. Finally add 1/4 cup boiling hot water (the water should be very hot; pour into the measuring cup immediately after boiling) and beat just until combined.
• Fill and bake: divide the batter among the prepared cupcake liners, filling each about 3/4 full. Bake in the preheated 350°F oven for 20–22 minutes, or until a toothpick inserted into the center comes out clean.
• Cool the cupcakes: let the cupcakes cool in the pan for 10 minutes, then transfer them to a wire rack to cool completely before frosting.
• Make the frosting: in a large bowl using an electric handheld mixer (or stand mixer), beat 8 ounces VERY soft cream cheese, 2 ounces VERY soft unsalted butter, and 1/4 teaspoon pure vanilla extract on medium-high speed until completely smooth, about 2 minutes. Reduce speed to low and gradually add 3 1/2 cups sifted confectioners' sugar and 1/4 teaspoon salt. Once all the sugar is incorporated, beat on high for 1–2 minutes. Add a little more confectioners' sugar only if needed to reach your preferred piping/spreading consistency.
• Frost and decorate: once the cupcakes are completely cool, spread or pipe the cream cheese frosting onto each cupcake. Immediately decorate with assorted sprinkles and nonpareils.
• Store: serve at once or store the frosted cupcakes covered in the refrigerator for up to 3 days.