Preheat oven to 350 degrees and line a cupcake pan with paper liners.
In a medium bowl, combine flour, baking powder, and salt. Set aside.
In a stand mixer or in a large bowl and a hand mixer, cream together butter and sugar for about 3-4 minutes, until light yellow and fluffy on medium/medium high speed.
Add the egg and vanilla and mix well.
In a small bowl or pourable measuring cup, whisk together sour cream and milk.
Starting with the dry ingredients, add about ⅓ to the butter mixture and beat until combined.
Add half of the milk/sour cream mixture and mix well. Continue alternating dry and wet ingredients, ending with the dry ingredients.
Divide the batter evenly between the cupcake tin. Bake for 14-16 minutes or until a toothpick inserted in the middle of the cupcake comes out clean.
Let cool in the pan for about 10 minutes and then transfer to a wire rack to cool the rest of the way.