Place butter, sugars, egg yolk, vanilla, salt, and espresso powder in the bowl of a stand mixer.
Mix until no more butter clumps remain.
Add in flour and mix until fully incorporated.
Mix in chocolate chunks.
Form dough into a roll that is 2” in diameter and wrap in plastic wrap.
Place in the freezer for 20 minutes or in the fridge for at least 2 hours. (You can store in the fridge for up to 2 days).
Preheat oven to 325 degrees f.
Slice into 1/3" rounds and place on parchment or baking mat lined baking sheet, 1" apart.
Bake for about 15 minutes or until edges just start to turn golden brown.
Allow to cool on baking sheet for about 5 minutes before removing.