1 Mix the dry ingredients: In a medium mixing bowl, combine the flour, milk powder, salt, and baking soda and whisk to mix. Set aside.
2 Beat the sugars and the butter: With an electric mixer or in a stand mixer with a beater attachment, beat together the granulated sugar, brown sugar, and softened butter on medium speed until combined, 30 to 60 seconds. Pick out any large pebbles of hard brown sugar that you see. Scrape down the sides of the bowl and the beaters.
3 Mix in the eggs and vanilla: Add the two eggs and vanilla to the butter-sugar mixture and beat at medium speed until no clumps or streaks remain, 10 to 20 seconds. Scrape down the sides of the bowl and the beaters.
4 Mix in the flour mixture: Add the flour mixture all at once. Beat on low speed just until no more dry streaks of flour are visible, 20 to 40 seconds. You may still see some flecks of milk powder — this is fine.
5 Mix in the chocolate chips: Add all of the chocolate chips and beat on low speed for just a few seconds until the chips are evenly incorporated.
6 Chill the dough: Scrape down the sides of the bowl and the beater. Cover the bowl and refrigerate for at least 30 minutes or up to three days. (Or freeze in individual scoops for up to 3 months.)
7 Heat the oven to 375F: Place a rack in the middle of the oven. Line two baking sheet with silicon baking mats or parchment.
I usually bake my cookies one sheet at a time to help them bake evenly. If you prefer, or are crunched for time, bake two sheets at a time with one sheet in the upper third of the oven and the other in the lower third of the oven.
8 Scoop the dough: Use a medium cookie scoop (or a well-mounded tablespoon measure) to scoop the dough out onto one of the baking sheet. Space the cookies about 2 inches apart.
9 Bake the cookies for 10 to 12 minutes: While the first batch bakes, scoop the cookies for the next batch.
The cookies are done when they look slightly puffed in the middle and are starting to turn toasty at the edges.
10 Cool the cookies: Cool the cookies on the baking sheet for about 5 minutes, or until the puffed middles collapse and the cookies have firmed a little. Transfer the cookies to a wire cooling rack to cool completely.
11 Continue baking cookies in batches until all the cookie dough has been used.
12 Enjoy! These cookies are at their best on the day they are made. Store leftovers in an airtight container; they will soften a little overnight but are still very delicious. They keep for about a week before becoming crumbly (at which point, I recommend crumbling them over ice cream!).