In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with an electric mixer on medium speed, cream butter and sugar until light and fluffy; about 3 minutes.
Add in eggs and vanilla and mix until thoroughly incorporated.
In a medium bowl, whisk together the flour, baking powder and salt.
Gradually add the flour mixture to the butter mixture until the dough comes together. Mix in sprinkles.
Cover and refrigerate dough for 1-2 hours.
When you’re ready to bake the cookies, preheat oven to 350°F and line baking sheets with parchment paper or a silicone baking liner.
Use a medium cookie scoop (about 1 1/2 tablespoons) to drop dough 2 inches apart onto the prepared cookie sheets.
Bake cookies in preheated oven for 10-12 minutes. Allow cookies to rest for a few minutes before removing them to a cooling rack to cool completely.
Once cookies have cooled completely, prepare the buttercream frosting.
For the Buttercream
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with an electric mixer, beat butter on medium-high speed for about 6 minutes.
With the mixer on low speed, slowly add in powdered sugar, and continue beating until well blended.
Add in vanilla, and 1 tablespoon milk. Mix on low speed until moistened.
Beat at high speed until frosting is smooth and fluffy; about 5 more minutes. If your frosting is too thick, gradually add a little more milk, 1 teaspoon at a time until you reach the desired consistency.
Pipe frosting onto cooled cookies and top with festive sprinkles.
If you want to maintain a bright, white frosting, use clear vanilla extract in the buttercream.