• Preheat the oven to 350℉. Line a cookie sheet with parchment paper. Set aside. 

Make The Funfetti Cookies

• In a large mixing bowl, add the dry funfetti cake mix, large eggs, vegetable oil, and vanilla extract. Use an electric hand mixer to beat on medium speed until the ingredients are thoroughly combined and a thick cookie dough forms.

  ^{*The cookie dough will be soft and slightly sticky, but if it's too soft to scoop put it in the fridge for 10-15 minutes. I normally always refrigerate it for a short time. It also helps the cookies not spread as much.}
  1 box (15.25 oz) funfetti cake mix, 2 large eggs, 1/3 cup vegetable oil, 1 teaspoon vanilla extract
• Using a medium cookie scoop (2 tablespoons), to drop the dough balls onto the prepared baking sheets spacing them about 2-inches apart.
• Bake for 11-14 minutes or until the edges are set and very lightly golden. Remove the cookies from the oven and allow them to cool on the cookie sheet for 5 minutes before transferring them to a wire cooling rack to cool completely.

Make The Buttercream Frosting

• In a medium bowl, beat the butter with an electric mixer until creamy.
  12 tablespoons (1½ sticks) unsalted butter
• Gradually add in the powdered sugar, a little bit at a time, and the vanilla extract and continue beating until smooth and combined well. 
  2¼ cups powdered sugar, 1 teaspoon vanilla extract
• Add the heavy cream in, a little bit at a time, until the frosting reaches a thick but spreadable consistency. 
  1-2 tablespoons heavy cream
• Once the cookies are completely cooled, use a piping bag with a large 1A tip, an offset spatula, or the back of a spoon to swirl a generous amount of buttercream frosting on top of each cookie in a circular motion.

  Immediatly after frosting, sprinkle your desired amount of rainbow sprinkles over the frosting. 
  rainbow sprinkles