In a large bowl using an electric mixer, stir sugar cookie mix, flour, butter, and egg on medium speed until soft dough forms.
Scoop dough into 2.5 teaspoon balls, or use a #60 disher to scoop balls of dough. Roll balls of dough into nonpareils/sprinkles. Place on an ungreased baking sheet or silicone mat/parchment paper lined baking sheet.
Press a ¼ teaspoon into the center of each dough ball creating a small indent. Indent will spread and become larger and shallower as cookies bake.
Bake for 8-10 minutes, or until edges are light golden brown. Cookies may appear to be soft and undone, but they will set as they cool. If your thumbprint indentation disappeared, repress ¼ teaspoon into the center immediately after removing from oven.
Cool cookies on cookie sheet for 1 minute; transfer to wire rack and cool completely.
For Vanilla Buttercream
In a large bowl using an electric mixer, cream butter, and shortening until creamy and smooth. Add vanilla and butter extracts; mix until combined.
Turn mixer on lowest speed and slowly add 1 lb of powdered sugar; mix until creamy. Scrape down the bowl as necessary. Adding the powdered sugar gets dusty - place a damp towel over the top of the mixer to shield flying powdered sugar dust.
Once all powdered sugar has been mixed in, add 1 tablespoon of milk or water at a time and mix. Continue to add and mix until desired consistency is achieved.
Once cookies have cooled completely, pipe a dollop of buttercream into the thumbprint indentation of each cookie. Eat & Enjoy!
Store Buttercream Thumbprint Sugar Cookies in an airtight container and will keep for 5+ days.
Buttercream Thumbprint Sugar Cookies | https://sarahsbakestudio.com/buttercream-thumbprint-sugar-cookies/