First, toast your whole milk powder! Spread milk powder in a non-stick frying pan and heat over medium low, stirring nearly-constantly with a rubber spatula, until the milk powder turns a light caramel color and starts to smell like caramel, then remove from heat and set aside.
Then brown your butter. In a small saucepan over medium-low heat, melt 1 cup plus 2 Tbsp (sometimes the butter comes out less than you started with when browning, you want 1 cup of browned butter) of salted butter and stir constantly until melted and simmering. Continue to simmer, on medium heat, for at least a few minutes, until the popping and hissing sounds have completely subsided (this is the water in the butter burning off – you’ll know once it’s silent). Continue to cook, stirring constantly, until the butter reaches a deep amber color and smells nutty or caramelly. It will start to foam over and the foam will rise to the top of the pan, so keep stirring/lifting up spoonfuls of the butter to see the color. Once it reaches that color, immediately take off the burner and pour into a small heat-proof bowl to cool. Once it's cooled for a few minutes, stir in your toasted whole milk powder and then refrigerate (or freeze) until solid but not totally hard - like the consistency of room temp butter.
Then, make your cookie dough! In a large bowl, using an electric mixer, beat the now solid browned butter + milk powder until smooth, then add in both the brown and white sugars and continue to cream/beat for 4-5 minutes until lighter, creamier, and fluffier. Then add in the vanilla and egg + egg yolks and beat until smooth and combined. Then, using a rubber spatula, stir in the flour, starch, salt, and baking powder until thick and stiffer like a dough. Finally, fold in your chopped milk and dark chocolate until distributed throughout the dough.
Now, the hard part: refrigerate the dough overnight or for at least 10 hours. Then remove from fridge and let thaw for 2-3 hours (it will be rock hard at first) until a bit softer and workable (it won't be totally soft). Preheat the oven to 350° Fahrenheit and line two cookie trays with parchment paper and then grease the parchment with cooking spray. Finally, scoop out scant 1/3 Cup of cookie dough - I recommend using a cookie scoop so you get uniform half-spheres of dough. I even roll the edges of the scoops so that your cookie edges will be smooth. Place six dough hunks on each cookie sheet (do NOT flatten them) and bake for 12-14 minutes (I recommend 12 - if you overbake your cookies they will not be chewy). Remove from oven and immediately sprinkle with flaky sea salt. Let cool COMPLETELY before removing from cookie sheets and eating - this will allow the cookies to settle, firm up, and be the perfectly chewy, buttery, delicious cookie that you want!