Preheat the oven to 350°F (177°C). Line a mini cupcake pan with paper liners. In a small bowl, whisk the flour, baking powder, and salt together until well-combined. Set aside.

Using a stand mixer or hand mixer, cream the butter and sugar until the mixture is very light and fluffy, about 3-4 minutes. Add the eggs one at a time until well-mixed. Then, stir in the sour cream, oil, and vanilla extract.

Pour half the flour mixture into the wet ingredients, followed by half of the milk. Mix on low just until combined. Repeat with remaining flour mixture and milk, stirring just until the batter comes together. Don't over-mix or the cupcakes will come out dense!

Using a #60 scoop, divide the batter evenly between 60 cupcake liners. They'll be about 2/3-3/4 of the way full. Bake the cupcakes for 11-12 minutes, rotating the pan halfway through. The cupcakes are ready when an inserted toothpick has just a few moist crumbs on it.

Let the cupcakes cool in the pan for about 5 minutes. Then, transfer the cupcakes to a wire rack to cool completely. Let the pan cool completely before you bake the rest of the cupcakes.

Add the butter and half of the powdered sugar to a large mixing bowl or the bowl of a stand mixer. Using a hand mixer or stand mixer with a whip attachment, beat the butter and powdered sugar until it's smooth. Add the rest of the powdered sugar, beating the mixture until it's well-combined. The frosting will be thick.

Add the salt, vanilla bean paste or extract, and whole milk. Whip the frosting on medium-high speed until it's very fluffy, 3-4 minutes. If the frosting is too thick, add more milk 1 teaspoon at a time, as needed.

Fit a piping bag with a decorative tip. Then, spoon the vanilla buttercream into the bag until it's about halfway full. Pipe the cooled cupcakes with the vanilla buttercream and top with sprinkles, if desired. Serve the cupcakes immediately or store them in an airtight container for up to 3 days. Enjoy!