In a bowl, whisk the flour, baking powder, baking soda, and salt together. Set aside.

In a large bowl using a hand mixer or stand mixer fitted with a paddle attachment, add the butter and add sugar. Cream butter & sugar on high speed until smooth and creamy, for about 2 minutes.

Scrape down the sides and up the bottom of the bowl with a rubber spatula. Give it a quick mix again to ensure there are no remaining pockets of unmixed ingredients.

Add egg, sour cream, and vanilla extract. Beat on medium-high speed until combined. If the mixture looks curdled, it's fine. The moisture from the sour cream may not bind with the butter at first.

Scrape down the sides and up the bottom of the bowl.

With the mixer on low speed, add the dry ingredients until combined. Stop mixing once all the flour is absorbed to avoid tough cookies. The dough/batter will be very thick, but creamy. Cover with plastic wrap and chill for 1 hour

Preheat the oven to 350F. Line 2 baking sheets with parchment or silicon marts. Bake for about 15 minutes. Rotate pans halfway through for even baking.

Allow the cookies to cool on the baking sheet for 3-4 minutes before transferring to a wire rack to cool completely.

While the cookies are cooling, add all of the frosting ingredients into a large bowl or into the bowl of a stand mixer fitted with a paddle attachment.

Decorate with sprinkles immediately after frosting each cookie (the icing may crust over quickly and the sprinkles won't stick).