Preheat the oven to 170 ° C for top and bottom heat. Grease the 12 cavities of the muffin baking pan or line them with paper liners.
Sift the flour twice. Add sugar, baking powder and salt and mix together.
Add the butter (cut into small pieces), sour cream, egg, egg yolk and vanilla and beat until frothy (about 30 seconds).
Spread the mixture in the muffin hollows. Do not fill in more than 2/3, otherwise the pages will overflow.
Bake until the tip is light golden, approx. 20 - 24 minutes, and put a toothpick in if it comes out clean. Let cool for 5 minutes and then remove with a knife. Let cool for about 45 minutes on a wire rack.
For the Vanilla Buttercream Frosting:
Beat the butter with a hand mixer until frothy (3-5 minutes). Sift the powdered sugar and mix in. You don`t have to use the whole 180 g of powdered sugar. Enter a little and season to taste.
Work in the cream or milk and vanilla and beat until creamy. If it is too dense, add more milk. If it`s too thin, either add more powdered sugar or flour. Usually I would use flour because it is sweet enough.