First, measure out and set the butter on the counter to soften roughtly 60 minutes before mixing the dough.
NO BROWN BUTTER POWDER? USE THESE TOASTING MILK POWDER INSTRUCTIONS: If you don't have Brown Butter Powder already, then you can make some at home by toasting ¼ cup (60g) of whole milk powder in a pan on low, to barely medium low temperature for 30-45 minutes.Just like a roux, keep moving the powder around the pan so it doesn’t burn. Breakup any pieces that begin to clump together as best as you can. The powder will go from an ivory color all the way to a toasted deep golden, light brown color. If too much of the milk powder is sticking to the bottom of the pan and you are having trouble scraping it up, turn your heat down or remove the pan from the burner to scrape and then return it to the heating element to continue. When you achieve a warm light brown color, remove from the burner and set aside to cool. Next, whisk together the flour, baking soda, and buttermilk powder. Then, set the flour mixture aside and measure out the sugars. Last, chop up your chocolate and set it aside.
Now, begin the cookie dough, by beating the butter on medium speed until it lightens and becomes fluffy. This will take between 3-5 minutes. Pour in the Brown Butter Powder (AKA toasted milk powder) and mix until it fully combines with the butter. Then, scrape down the bowl and pour in the brown and granulated sugars. Continue beating for another 2-3 minutes. The mixture will go from very grainy to mostly smooth and uniform in color and texture.
At this point, scrape the butter and sugar mixture down the sides of the bowl again and turn the mixer to low. Add the egg + plus yolk and mix until it’s fully incorporated. Then, add in the salt and vanilla extract and continue beating for about 30 seconds, to make sure they are mixed in pretty well.
Next, pour in the flour mixture in small increments, to keep it from puffing out of the mixing bowl. As soon as the flour mixes in, stop the machine. Do NOT overmix. The dough should be very soft. If you overmix, once the flour is added, the gluten in the flour will toughen the dough. With a sturdy spatula, mix in the chocolate chunks until just combined.
Grab a #24, 2in (5cm) cookie scoop (it should hold roughly 2½ tbsp or 45-50 g of dough). Scoop cookies onto a silicone mat lined baking sheet and cover them with your prefered cling wrap. Chill the dough for 6+ hours. For the best possible results, chill for 24-48 hours. (See notes on chill times)
After resting the dough, preheat your oven to 375°F (190°C). When it comes to temperature, arrange your cookies roughly 1½-2 in (3.8-5 cm) apart and place them in the oven. As soon as they are in, turn the temperature down to 350°F (177°C) and bake for 10-12 minutes. The edges of the cookies should just start to brown and the middle should be completely puffed and not look wet. Let cool for 5 minutes before moving to a cooling rack.