Preheat oven to 350°F. Line a 24-capacity mini muffin tin with mini cupcake paper liners. Set aside.

Combine the cake flour, baking powder, salt, and baking soda in a small bowl. Set aside.

In a large mixing bowl using a handheld mixer on medium speed, beat together the granulated sugar and softened unsalted butter until light and fluffy.

Add the egg and beat until fully incorporated.

Add the sour cream and vanilla extract and beat just until combined.

Add half the flour mixture to the bowl of wet ingredients and beat on low just until the flour is fully incorporated. Add the milk, then the remaining flour mixture. Be sure to incorporate each addition fully before adding the next. You will need to scrape down the sides and bottom of the bowl to ensure that all the batter has been evenly mixed.

Using a small cookie scoop, scoop out about 1 tablespoon of cupcake batter and fill each paper liner ⅔ full.

Bake the mini cupcakes for 11-13 minutes or until lightly golden on the edges and a toothpick inserted into the center comes out clean.

Allow the cupcakes to cool in the muffin pan for 5 minutes before transferring them to a wire rack to cool completely on the counter.

While your mini cupcakes are cooling, you can make your buttercream frosting. In a large mixing bowl, beat on medium speed the softened unsalted butter and salt for 1 minute or until light and fluffy.

Add 1 cup of the sifted powdered sugar to the beaten butter, and beat on medium-high speed until smooth and fluffy.

Add the remaining 1 cup of sifted powdered sugar and beat again until smooth.

Add the vanilla extract and heavy cream to the buttercream frosting. Beat on medium-high speed until your frosting is smooth, light, and fluffy.

Transfer your buttercream frosting to a piping bag fitted with a piping tip, and frost each of your cooled cupcakes. You can garnish your cupcakes with sprinkles before plating and serving them.