First, measure out and set your butter on the counter to soften. Then, toast the milk powder in a pan on low, to medium low temperature for about 1 hour. 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.
When the toasted powder is about room temperature, whisk together the flour, baking soda, toasted milk powder, and buttermilk powder. Set the flour mixture aside and measure out the sugars. At this point, you can also chop up your chocolate and set it aside.
Begin the cookie dough, by beating the butter on medium speed until it lightens and becomes fluffy. This will take between 3-5 minutes. At that point, add both the brown and granulated sugars and continue beating for another 2-3 minutes. The mixture will go from very grainy to mostly uniform in color and texture (though it won’t be completely smooth).
At this point, scrape the butter and sugar mixture down the sides of the bowl 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 the dough it will toughen. 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 best results. (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.