Preheat oven to 350°F (180°C). Line a 12-cup muffin tin.
Combine milk, oil, vanilla, and vinegar.
Stir sugar, flour, and baking powder in a separate bowl.
Pour wet into dry. Whisk gently until smooth.
Divide into liners (batter is runny). Bake 20–25 min until a toothpick comes out clean. Cool completely.
Buttercream: Beat butter + shortening until smooth. Add sugar, vanilla, and milk. Beat until fluffy.
Pipe or spread frosting. Add sprinkles if you like.
Store in an airtight container, cool place, up to 3 days.
