Preheat the oven to 350°F (177°C). Line a 12-cup muffin pan with cupcake liners. This recipe yields 14 cupcakes, so line a second muffin pan with 2 cupcake liners, or bake in batches.

In a large bowl, whisk the flour, baking powder, baking soda, and salt together. Set aside. In a medium bowl, whisk the melted butter and sugar together. The mixture will be gritty. Refrigerate for 5 minutes to cool and slightly thicken.

Whisk in the egg, sour cream, and vanilla extract until combined, then whisk in the milk.

Pour the wet ingredients into the dry ingredients and whisk until the batter is completely combined. The batter should be creamy and mostly smooth; a few small lumps are OK. Fold in the sprinkles.

Pour/spoon the batter into the liners, filling only 3/4 full to avoid spilling over the sides. You should have enough batter for 14 cupcakes.

Bake for 20–22 minutes, or until a toothpick inserted in the center comes out clean. Do not over-bake.

Remove cupcakes from the oven and allow them to cool in the pan for 10 minutes, and then transfer to a wire rack to cool completely. Cupcakes must be completely cooled before decorating.

With a handheld or stand mixer fitted with a paddle attachment, beat the butter and cream cheese together on medium-high speed until creamy and smooth, about 2 minutes. Add the confectioners’ sugar, heavy cream/milk, and vanilla extract. Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 2 full minutes. Taste. Add a pinch of salt if the frosting is too sweet. I always add 1/8 teaspoon. Add up to 1/2 cup more confectioners’ sugar if frosting is too thin, or more cream/milk—a Tablespoon at a time—if frosting is too thick.

Frost cooled cupcakes and top with sprinkles, if using. You can swipe the frosting on with an icing knife or use a piping tip such as Ateco 826 or Wilton 1M.

Store cupcakes in an airtight container at room temperature for up to 3 days and in the refrigerator up to 5. I recommend a cupcake carrier for storing and transporting decorated cupcakes.