Preheat oven to 350°F. Line an 8-inch round cake pan with parchment. In a large bowl, whisk together flour, sugar, baking powder, and salt.

Add buttermilk, oil, egg, and vanilla. Stir just until smooth, about 1 minute.

Scrape batter into the pan and smooth the top. Bake 25 to 28 minutes, until a toothpick comes out with a few moist crumbs. Cool in the pan 10 minutes, then turn onto a rack to cool completely.

For the frosting, beat butter until creamy. Add powdered sugar, vanilla, and milk. Beat on low to combine, then on high for 2 to 3 minutes until fluffy. Adjust with a little milk or sugar as needed.

Once the cake is cool, spread the frosting over the top. Add sprinkles if you like, then slice and serve.