Even Cupid would fall in love with these classic vanilla cupcakes topped with buttercream frosting. If you're short on time, order plain cupcakes from your local bakery – either way, kids will have a great time decorating their own special treat.
Preheat oven to 350 degrees. Line a 12-cup muffin tin with foil or paper liners.
Whisk together flour, baking powder and salt; set aside.
Using an electric mixer, beat together sugar and butter on medium-high until light and fluffy (about 3 minutes).
Reduce speed to low and add egg and egg white, one at a time; beat in vanilla.
Alternate adding 1/3 flour mixture with 1/2 of milk until both are all in. Scrape down sides of bowl.
Increase speed to medium-high until flour is fully incorporated (about 30 seconds). Fill muffin cups about 3/4 full.
Bake until light golden and a toothpick inserted in the center comes out clean (18-20 minutes).
Allow pan to cool for 5 minutes. Transfer cupcakes to wire rack; let cool for an hour.
Make sure all ingredients are room temperature. Using an electric mixer, blend all ingredients on low speed until combined (about 1 minute). Scrape down sides of bowl.
Beat on medium speed until fluffy (about 3 minutes). If frosting is dry, add more milk, 1 teaspoon at a time.
Pipe frosting onto cupcakes using pastry bag fitted with a large-star piping tip.
Sprinkle cupcakes with mini hearts or sprinkles.
Slightly cut into the top of each grapefruit triangle to create a heart shape. (If you can't find triangle-shaped grapefruit candy, use strawberries – simply slice them from top to bottom to create a heart shape). Place atop each cupcake.