In a large-size bowl using an electric handheld mixer on medium speed, beat together the melted butter, eggs, and vanilla until combined.

Sift in the cake mix and cornstarch and mix on low speed until the dry ingredients are well combined - the dough will be very soft. Fold in the sprinkles, being careful not to overmix and cause the sprinkles to bleed. Cover the dough and refrigerate for 1 hour.

Preheat the oven to 350℉. Line 2 baking sheets with parchment paper. When done chilling, roll the dough into balls, using about 1 ½ tablespoons of dough per cookie. Place on the prepared pans, 12 cookies per pan.

Bake the cookies one pan at a time in the preheated oven for 10-12 minutes or until golden brown around the edges. Let cool on the pan for 5 minutes before transferring to a wire rack to finish cooling completely before sandwiching with the frosting. Bake the remaining cookies.

In a medium bowl using an electric handheld mixer on medium speed, whip the softened butter until fluffy. Add the powdered sugar and almond extract and beat on low speed until well combined. Add 1 tablespoon of milk or more if the texture is too thick. Pipe or spread with a knife, about 1 tablespoon of frosting per cookie onto the cooled cookies. Enjoy!