1) Prep
Heat oven to 350°F. Set a rack in the center of the oven. Line 2 sheet pans with parchment.
If your oven runs small or hot, bake one pan at a time. If you bake two at once, keep them on separate racks and rotate and swap positions halfway through.
2) Mix the dry ingredients
In a bowl, whisk together flour, cocoa powder, baking soda, and salt.
3) Build the dough
In a larger bowl, whisk melted butter with brown sugar and granulated sugar until glossy, about 30 seconds. The butter should be warm, not hot to the touch so it does not turn your dough into a spread-fest.
Whisk in the egg, egg yolk, and vanilla. Fold in the dry ingredients with a spatula until you have a thick dough, then fold in the chocolate chips.
4) Chill
Cover the bowl and chill the dough for 20 to 30 minutes. This is the difference between “nice thick cookies” and “why are these the size of coasters.”
5) Make the cocoa swirl paste
In a small bowl, stir cocoa powder and powdered sugar. Add milk or water a tablespoon at a time until you have a thick, spoonable paste that holds a line when dragged with a spoon.
Too runny? Add a pinch more cocoa or powdered sugar. Too thick? Add liquid a few drops at a time until it behaves.
6) Scoop and swirl
Scoop dough into 2 tablespoon portions (about 35 to 40 g each) and place 2 inches apart on the pans. Yield depends on scoop size, but you should get about 18 cookies.
Add about 1/4 teaspoon cocoa paste to the top of each dough ball. Use a toothpick, skewer, or the tip of a butter knife to swirl it into a circle. Do not overthink it. Imperfect circles look cooler.
7) Bake
Bake 10 to 12 minutes, rotating the pan halfway through, until the edges look set and the centers still look slightly soft.
8) Cool and finish
Cool on the pan for 5 minutes, then transfer to a rack. If you like, add a pinch of flaky salt while they are still warm.
