Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.

In a medium bowl, whisk together the sieved all-purpose flour, baking powder, and salt. Set aside.

In a large bowl, using an electric mixer, cream together the softened unsalted butter and granulated sugar on medium-high speed until light and fluffy, about 3-5 minutes.

Add the eggs one at a time, beating well after each addition until fully incorporated. Scrape down the sides of the bowl as needed. Beat in the vanilla extract.

With the mixer on low speed, gradually add about one-third of the dry ingredients to the wet ingredients, mixing until just combined.

Pour in half of the buttermilk and mix until just combined. Repeat with another third of the dry ingredients, the remaining buttermilk, and finally the last third of the dry ingredients. Mix until just combined and no streaks of flour remain, being careful not to overmix.

Gently fold in the 1/2 cup of rainbow sprinkles with a spatula until evenly distributed.

Divide the batter evenly among the 12 cupcake liners, filling each about two-thirds full.

Bake for 18-20 minutes, or until a wooden skewer inserted into the center of a cupcake comes out clean.

Remove cupcakes from the oven and let them cool in the muffin tin for 5 minutes before transferring them to a wire rack to cool completely. Cupcakes must be completely cool before frosting.

In a large bowl, using an electric mixer, beat the softened unsalted butter on medium speed until creamy, about 2 minutes.

Gradually add the sieved powdered sugar, one cup at a time, beating on low speed until combined, then increase to medium-high speed and beat until smooth.

Add the cold heavy cream, vanilla extract, and pinch of salt. Beat on high speed for another 3-5 minutes until the frosting is light, fluffy, and smooth. If the frosting is too thick, add more cream (1 tsp at a time); if too thin, add more powdered sugar (1 tbsp at a time).

Once cupcakes are completely cool, frost them generously with the vanilla buttercream using an offset spatula or a piping bag with your favorite tip.

Sprinkle the remaining 2 tablespoons of rainbow sprinkles over the frosting before serving.