Preheat the oven to 350°F. Line a muffin tin with liners, set aside.

In a microwave-safe bowl, heat butter in 30-second increments until melted. Set aside, cooling until just warm.

In a small bowl, gently whisk together the sour cream and buttermilk. Set aside.

Alternating between wet and dry ingredients, add half of the sour cream/buttermilk mixture and half of the flour mixture to the mixer, mixing on medium-low speed after each addition until just combined. Repeat this step twice. Be careful not to overmix. Scrape down the sides of the bowl.

Bake for about 16 minutes, or until the tops are very lightly golden brown and a toothpick inserted in the center comes out with a few moist crumbs. Let cool 5 minutes in the pan, then remove to cooling rack to cool to room temperature before frosting.

While the cupcakes are baking, in the bowl of a stand mixer fitted with the paddle attachment, add the butter and cream cheese. Mix together on medium-high speed until light, creamy and smooth, about 3 minutes. Scrape down the bowl, then add in the vanilla and salt and mix until just combined.

Add in the sifted powdered sugar one cup at a time, scraping down the sides of the bowl to prevent any clumps. Beat on medium-high speed for about 1 minute, until light and fluffy. If you need a stiffer consistency for decorative piping, add more powdered sugar. If your frosting is too thick, add a splash of milk or heavy cream and beat until combined.

Pipe the frosting onto the cupcakes, or use an offset spatula to spread the frosting onto each cupcake, then top with rainbow sprinkles. Store in an airtight container at room temperature for up to 1 day, then place in the fridge for up to 3 days, letting come to room temperature before serving.