Cookies

1 cup unsalted butter, softened 

1 1/2 cups granulated sugar

1 egg + 1 egg yolk

2 teaspoons vanilla extract

3 cups all-purpose flour 

1 1/4 teaspoons baking soda

1/2 teaspoon cream of tartar

1 teaspoon cornstarch

1/2 teaspoon salt

1 cup sprinkles

Frosting

3/4 cup unsalted butter, softened

2 1/4 cups confectioners’ sugar

3 Tablespoons heavy cream

1 teaspoon vanilla extract

1/8 teaspoon salt

For the cookies: 

In a large bowl using a hand-held mixer or stand mixer fitted with a paddle attachment, cream the softened butter for about 1 minute on medium speed. Get it nice and smooth, then add the sugar on medium speed and beat until fluffy and light in color. Beat in egg, egg yolk, and vanilla. Scrape down the sides and up the bottom of the bowl as needed.  In a medium bowl, whisk the flour, baking soda, cream of tartar, cornstarch, and salt together until combined. 

With the mixer running on low speed, slowly add the dry ingredients to the wet ingredients in 3 different parts.  Once combined, gently beat in the sprinkles on low speed until evenly dispersed. Cover the cookie dough tightly and chill in the refrigerator for at least 3 hours and up to 3 days. Chilling is mandatory. Remove cookie dough from the refrigerator and let sit out at room temperature for at least 15 minutes.  Preheat the oven to 325° and line 2 large cookie sheets with parchment paper.  

Roll cookie dough into balls, about 1.5 Tablespoons of dough in each ball, and set on a cookie sheet– about 8-9 cookies per sheet. Bake in batches for 11-12 minutes or until the edges are slightly browned. The cookies will still appear very soft in the centers. Allow the cookies to cool on the cookie sheets for 3 minutes, then move to a wire rack to cool completely.  

For the frosting: 

In a large bowl using a hand-held mixer or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy – about 2 minutes. Add confectioners’ sugar, cream, vanilla extract, and salt with the mixer running on low. Increase to high speed and beat for 3 full minutes. Add more confectioners’ sugar if frosting is too thin, more cream if frosting is too thick, or a pinch more of salt if frosting is way too sweet.