Start by beating the butter and sugar together until light and fluffy. This step incorporates air into the dough, giving the cookies a soft texture. Add vanilla and egg, then mix until smooth.
On low speed, combine flour, cornstarch, baking powder, baking soda, and salt into the wet mixture. Keep mixing until you have a soft, cohesive dough. Avoid overmixing to prevent tough cookies.
Scoop dough into balls using your cookie scoop, then roll in sprinkles until fully coated. Place on a lined baking sheet, cover, and chill for 30 minutes. Chilling helps the cookies hold their shape while baking.
Preheat your oven to 350°F (175°C). Place the chilled cookies on baking sheets about 2 inches apart. Bake for 10 minutes or until the edges are set but the centers remain soft.
Right after baking, use the back of a tablespoon to gently press the center of each cookie. This creates a perfect cavity for the frosting once the cookies have cooled completely.
Beat butter until smooth, then add powdered sugar, cream, vanilla, and salt. Mix until fluffy. Tint with food coloring if desired for a fun, colorful presentation.
Once cooled, pipe buttercream into each well. Sprinkle a few extra sprinkles on top for an extra festive touch.