Preheat your oven to 350°F (175°C). Grease and line two 4-inch pans or one 6-inch round pan with parchment paper.

In a large bowl, cream the butter and sugar together for 2–3 minutes until the mixture is fluffy.

Add the egg and vanilla extract to the bowl, and beat everything until well combined.

In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, alternately with the whole milk, mixing gently. If you're using sour cream, fold it in at this stage.

Divide the batter evenly between the prepared pans. Bake for 18–22 minutes, or until a toothpick inserted into the center comes out clean.

Let the cakes cool in the pans for 5–10 minutes, then transfer them to a wire rack to cool completely.

For the frosting, beat the butter until smooth. Gradually add the powdered sugar, mixing well.

Then add the vanilla and cream or milk, beating until fluffy.

Once the cakes are cool, frost and decorate them with colored frosting and sprinkles.