In a large bowl, cream together butter and sugar until fluffy, about 2 minutes. Add vanilla, almond extract and the egg. Mix until well combined.
1 cup butter, 1 1/4 cup granulated sugar, 2 teaspoons vanilla extract, 1 teaspoon almond extract, 1 whole large egg
In a separate bowl, sift the flour and cornstarch together. Add the baking soda and salt. With the mixer on low speed, slowly pour in the flour. Stir until combined.
Using a spatula, mix in the sprinkles. Be careful not to over mix, or the sprinkles will break and the color will bleed.
1/2 cup rainbow sprinkles
Using a small cookie scoop (1 Tablespoon), scoop balls of dough onto a baking sheet. Chill for 30 minutes.
Preheat oven to 375°F.
Gently press down on the cookies, this helps to make sure they have a slightly flat surface when making the sandwich cookies. Bake cookies for 9-11 minutes, until the edges are set and the tops appear dry. Remove from the oven and let cool for a few minutes on on the baking tray, then remove to a wire rack to cool completely.
While the cookies are cooling you can make the frosting. Cream everything together until you get the desired consistency. You might have to adjust the milk and powdered sugar slightly to make it soft and spreadable.
Once the cookies are completely cool, match up pairs of cookies. Spread about 1 tablespoon of frosting on one of the cookies. Place the other on top, and slightly press down.
Roll the outside edge of the cookies in the additional sprinkles. Let the cookies sit for about an hour for the frosting to set up.