2cupsall-purpose flour (be sure NOT to pack your flour)
1/2cupfull-fat sour cream
2large eggs plus one large egg yolk, at room temperature
1/4cupboiling hot water (should be very hot, and poured into measuring cup right after boiled)
For the Cream Cheese Frosting:
8ouncescream cheese, VERY soft
2ouncesunsalted butter, VERY soft
1/4teaspoonpure vanilla extract
3and 1/2 cups confectioners' sugar, sifted (more if needed to reach preferred consistency)
Assorted sprinkles and nonpareils
To brown the butter:
Place the butter in a small sauce pan over medium-high heat and cook for about 3 minutes – stirring almost constantly – until the butter has turned an amber gold/brown and smells slightly nutty. Remove from heat and pour the butter into the mixing bowl you'll be making the cake batter in. Place the bowl in the refrigerator or freezer and bring the butter back to room temperature. You’ll know the butter is at room temperature when you press a finger into the top and it makes a slight indentation. It should be solid but soft.
For the cupcakes:
Preheat oven to 350 degrees (F). Line a 12-mold cupcake tin with paper liners and lightly spray the liners with non-stick spray, set aside. In a large bowl combine the flour, baking powder and baking soda. Add sprinkles and toss to coat; set aside. In a small bowl combine the milk and sour cream, mix well to combine and set aside. In a large bowl using a handheld electric mixer (or a stand mixer fitted with the paddle attachment) beat the butter and sugar until light and fluffy; about 4 minutes (don't skimp on time here!). Beat in the eggs and egg yolk, one at a time, beating well after each addition. Reduce mixer speed to low and gradually add in flour mixture in 2 additions, alternating with sour cream/milk mixture, beginning and ending with the flour. Finally stir in the hot water, beating just until combined.
Fill the prepared cupcake tins 3/4 of the way full. Bake in preheated oven for 20-22 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Allow cupcakes to cool in the pan for 10 minutes before transferring them to a cooling rack to cool completely. Set aside and make the frosting.
For the cream cheese frosting:
In a large bowl using an electric handheld mixer, beat the cream cheese, butter, and vanilla on medium-high speed until completely smooth; about 2 minutes. Reduce the speed to low and gradually add the sifted confectioners' sugar and salt. Once all of the sugar has been added beat on high-speed for 1-2 minutes. Once the cupcakes have completely cooled, spread the frosting over the cupcakes or use a piping bag to pipe it into decorative swirls. Decorate at once with sprinkles and nonpareils. Serve at once or store the cupcakes in the fridge, covered, for up to 3 days.
I suggest doubling the frosting if you plan to pipe it on in high swirls. Cupcakes will be slightly darker on the lids due to the brown butter; the interiors will be soft and fluffy.