Melt the butter in a large microwave-safe bowl. Let it cool to room temperature—don’t add sugars while it’s still hot or the cookies will turn out greasy and spread too much.
Once butter has cooled, add both sugars. Use a whisk to briskly mix until combined. Add vanilla extract and egg yolk. Whisk until smooth and emulsified (it should look creamy).
Whisk in the baking soda and salt. Then switch to a wooden spoon and stir in the flour. Mix just until no dry streaks remain.
Add the chopped chocolate and chocolate chips. Mix to combine—knead the chips into the dough with your hands if needed. Divide dough into 9 equal portions (about 2 tablespoons or 38 grams each). Roll each into a ball that’s taller rather than wide (the shape of a mini cylinder).
Place the dough balls on a parchment-lined sheet pan. Cover and refrigerate for 30 minutes up to 1 hour.
Preheat the oven to 325°F. Arrange 6 dough balls on a silicone-lined baking sheet with plenty of space in between. Bake for 8–12 minutes or until edges are just golden and tops look set (not glossy). Don't overbake!
Remove from the oven and, while still hot, use a metal spatula to gently press any uneven edges in toward the center. Press a few extra chocolate chips on top if desired. Let the cookies cool on the pan for 5 minutes before transferring to a wire rack.
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Notes
Note 1: Egg yolk must be from a large egg. Save the white for another recipe or discard.Note 2: I like using chopped dark chocolate bars with milk chocolate chips for contrast. You can use all chips if you prefer. High-quality chocolate for the best cookie flavor! Use whatever type of chocolate you like best: milk, semi-sweet, dark, or white.Storage: Room temperature: Store cookies in an airtight container for up to 4 days. Freeze: Freeze raw dough balls on a lined sheet until solid. Transfer to a freezer-safe bag. Bake from frozen—just add an extra minute or two.