In a large bowl, beat together melted butter, sugar, egg, sour cream, milk, and vanilla until combined. Set aside
In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
Add sprinkles to wet ingredients.
Gradually stir dry ingredients into wet ingredients until lumps are gone.
Transfer batter into prepared cupcake liners.
Bake for up to 20-25 minutes or until a toothpick inserted in the middle comes out clean.
Allow cupcakes to cool.
In a medium bowl, whip butter until fluffy.
Gradually add powdered sugar until you reach desired consistency.
If the mixture is still stiff, gradually add milk.
Transfer frosting to a piping bag and frost cupcakes.
Sprinkle each with sprinkles.
Notes
Kept in an airtight container on the counter, these homemade cupcakes are best if enjoyed within 1-2 days. They’ll last longer kept in the fridge- about a week.