Toast the milk powder at 150 °C for 30 min until it turns yellow. For heavier dulce de leche taste, you can go a bit browner.
Soften the butter with your whisk.
Scale brown, white sugars together and whisk it with butter. Whisk until the mixture is combined and slightly lightened, but still a bit gritty. You don't want your sugar to fully dissolve yet.
Pour the liquid (egg, salt, and vanilla extract) into the bowl and add it in three. Whisk thoroughly after each addition to completely incorporate the liquid before adding the next.
Sift the dry ingredients(flour, milk powder, baking soda, baking powder) into your batter.
Start folding gendly. When the flour is about 60% incorporated, where you can still see white flour here and there, dump in your chocolate cihps.
Continue folding until you see no white flour in the batter. Do not overmix! Overmixing will create too much gluten and will make your cookies tough. It will also cause cookies to spread too much.
Divide the dough equally into 6 round balls(about 100g.)
Wrap them and freeze for at least 40 minutes. Meanwhile, start preheating your oven at 170°C/338°F.
Lay out your cookies on your pan. These are large cookies so please make sure you give them some room. Flatten them just a little bit with your palm.
Optional: Sprinkle maldon salt on top for salty flavour.
Bake your cookies at 160 °C/320°F for 12~13 minutes. Check the colors before you take them out. If the edges are brown, but the center is still pretty light, I take them out because cookies keep baking even after you take them out from the oven. This is due to the heat from your pan. Enjoy!