Preheat oven to 350°F. Cream butter and sugar until light and fluffy. Add the yogurt and vanilla. Add eggs one at a time. Add flour, cornstarch, baking powder, and salt until everything is incorporated. Roll dough into 1 1/2" balls and flatten slightly with the bottom of a glass. Place on an ungreased cookie sheet Bake for 10 minutes, cool on wire racks.

Beat together the butter, sugar, vanilla, and the milk until very smooth. Spread or pipe onto cooled cookies and immediately add sprinkles. Store in an air tight container.