First, preheat oven to 350 degrees Fahrenheit. Line a baking sheet with parchment paper.
In the bowl of a stand mixer, add cold butter and sugar. Mix on high for 2 to 3 minutes or until fluffy. There should be no clumps of butter remaining.
Then, add in egg, egg yolk and vanilla. Stir to combine.
Add all remaining ingredients. Stir to combine.
Then, use a cookie scoop to scoop balls of dough onto lined pan. Use hands to slightly flatten each ball of dough.
Bake for 10 to 12 minutes or until cookies are lightly golden around the edges.
Finally, remove from oven. Allow cookies to cool on pan for 10 minutes before carefully removing to a cooling rack.
Notes
Use cold butter right from the fridge. Do not use softened or melted butter.
The cold butter prevents the cookies from spreading too thin.
Beat butter and sugar until no clumps of butter remain. If needed, stop mixing and scrape down sides of bowl with spatula. Then continue mixing.
The cookie dough batter will be pretty thick.
These cookies do not spread much while baking. Therefore, flatten each ball of dough slightly before baking.
Store leftovers in an airtight container for up to 4 days at room temperature.