Add the softened butter and granulated sugar to the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed until light and fluffy– about 1 minute. 

Next, add the egg yolk and vanilla extract. Mix until combined. Scrape down the sides of the bowl if needed. 

With the mixer on low speed, slowly add the flour and baking soda. Mix just until no large lumps of flour remain– you don’t want to overmix! The dough will look dry but should hold together when you pinch it. If it does not, add 1-2 tablespoons of milk and mix again. 

If using sprinkles, add them now and gently fold them into the cookie dough. I recommend a rubber spatula for this– using the mixer can break up the sprinkles too much.

Roll the dough into a large log (about 1.5” wide and 12” long). Wrap the dough in parchment or plastic and place it in the fridge to chill for at least 2 hours. At this point, the dough can be placed in a large resealable bag and kept in the fridge for up to three days or in the freezer for up to two months. 

When you’re ready to bake, preheat the oven to 350F and line two large baking sheets with parchment paper. 

Slice the log into ¼” thick rounds and place them at least 2” apart on the baking sheet. 

Bake for 8-10 minutes, or until the cookies are just beginning to turn golden brown on the bottom. 

Allow the cookies to cool completely, and then transfer them to an airtight container. 

Storage: Store in an airtight container at room temperature for up to two days.