In a large bowl using a hand or stand mixer with the paddle attachment, mix the butter, cream cheese, and sugar on medium speed until light and fluffy, about 1 minute.

Scrape down the bowl and mix in the egg, egg yolk, and vanilla until smooth.

Scrape down the bowl and mix in the salt, baking powder, and cream of tartar until combined.

Last, mix in the flour. When your mixer starts to struggle with the thick dough, remove from the mixer and use a rubber spatula to ensure the dough is well combined. It’s VERY thick and sticky, so this requires a little elbow grease!

Dump the dough out onto a sheet of plastic wrap lightly dusted with flour.

Coat your hands in flour and pat the dough out into a 1” thick rectangle. Wrap and chill for at least 1 hour or overnight.

When ready, preheat the oven to 350F and line a large baking sheet with parchment paper.

Roll the dough out onto a floured surface to ¼” thick (we want nice and thick cookies).

Cut out as many cookies as you can and place them about an inch apart on the baking sheet. With any leftover cut out cookies, store in the refrigerator.

Bake for 8-10 minutes (9 was the sweet spot for me), or until they look dry on the surface but just barely done in the middle. In the meantime, you can re-roll the dough scraps, continuing to store any cut out cookies in the refrigerator until it’s time to bake them.

Let the cookies rest on the pan for about a minute, then transfer to a cooling rack before baking the rest of the batch.

In a large bowl using a hand or stand mixer with the paddle attachment, mix the butter and cream cheese until smooth.

Mix in the powdered sugar one cup at a time, followed by the vanilla. It should be thick and smooth. Mix in a few drops of gel food coloring if desired.

Spread the frosting on the cooled cookies and immediately top with sprinkles. If you wait too long, the sprinkles won’t stick so frost one cookie, add sprinkles, then move on to the next.