In a large microwave-safe bowl, partially melt the butter in a microwave (about 35 seconds straight from the fridge, just enough to easily mix it). Let butter stand until it has cooled to room temperature. Do not add sugars to hot butter, or the cookies will be greasy and spread.
Once butter has cooled to room temperature, add brown sugar and granulated sugar. Stir until smooth and combined. Add vanilla extract and egg yolk. Stir to combine.
Add baking soda and flour. Stir until a thick dough forms.
Coarsely chop the chocolate bar and add in the chunks and chocolate chips. Stir until combined; you may need to knead the chips into the dough. Divide dough evenly into 9 parts; each cookie is about 2 tablespoons (38 grams). Roll dough into cookie balls that are tall and cylindrical (as opposed to short and round—see photos in post). Place dough balls on a parchment-lined sheet pan, cover, and chill for 30 minutes to an hour.
When ready to bake, set the oven temperature to 325°F. Line a sheet pan with parchment paper or a silicone liner. Place dough balls on sheet pan with plenty of space between cookies—I only bake 6 at a time. Bake 8–12 minutes or until very lightly golden brown at the edges and no longer glossy on top.
Remove cookies from oven and let stand on the sheet pan for 3–4 minutes before removing with a metal spatula to a cooling rack to finish cooling. I like to press a few more chocolate chips into the tops of the cookies right out of the oven (optional).
Video
Notes
Note 1: Egg yolk must be large. Reserve the white for another recipe or discard.Note 2: For the chocolate, I chop a milk chocolate bar and add chocolate chips for added texture/flavor. You can use only chocolate chips instead of the chocolate bar and chips. Use your favorite type of chocolate—semi-sweet, dark, or milk. I like milk chocolate chips paired with a dark chocolate bar. High-quality chocolate makes a big difference in the flavor and texture of the cookies.Storage: Store cookies in an airtight container at room temperature for up to 4 days. For freezing, freeze dough balls on a parchment-lined sheet pan until solid, then freeze dough balls in a freezer-safe bag.