Brown Butter Oatmeal Chocolate Chip Cookies

Method

1. Brown the butter: Put the 170 g butter in a light-colored saucepan over medium heat. Once melted, add the milk powder if using. Cook, stirring and scraping the bottom constantly as the butter foams. Continue until the milk solids turn a deep golden-brown, the aroma is nutty and toffee-like, and the sizzling begins to quiet, about 5 to 8 minutes. Immediately scrape every bit of the browned butter and toasted milk solids into a large heatproof mixing bowl so they do not burn.
2. Restore the moisture and cool the butter: Add the remaining 56 g butter to the hot brown butter and whisk until melted, then whisk in 25 g whole milk or 25 g chilled brewed espresso. For the fastest same-night method, set the mixing bowl into an ice bath and whisk or stir continuously until the butter mixture turns opaque, thickens, and reaches the consistency of very soft room-temperature butter. Remove it from the ice bath before it becomes firm or lumpy. Without an ice bath, let it cool at room temperature and then chill briefly as needed until softly set rather than liquid.
3. Mix the dry ingredients: In a medium bowl, whisk together the 190 g flour, 180 g rolled oats, 8 g cornstarch, 5 g baking soda, 4 g fine sea salt, and 1.5 g cinnamon until evenly combined.
4. Cream the butter and brown sugar: Add the 265 g light brown sugar to the softly set butter mixture. Whisk vigorously or beat on medium speed for about 60 to 90 seconds, until smooth, slightly lightened, and modestly aerated. Do not whip it as fluffy as cake batter. Add the whole egg, egg yolk, and vanilla and beat for about 45 to 60 seconds more, until glossy and fully emulsified.
5. Make the oatmeal dough: Add the dry mixture to the wet mixture and fold with a rubber spatula just until the dry ingredients are almost incorporated. Stop as soon as no large dry pockets remain; do not keep mixing simply to make the dough perfectly smooth.
6. Add the chocolate: Fold in the 250 g semisweet chocolate just until evenly distributed. The dough should be thick, soft, and slightly tacky.
7. Portion and chill: Portion the dough into about 55 g mounds. A No. 24 disher works especially well: pack the scoop and leave it slightly heaped above the rim, then check the first few portions on a scale because disher capacities vary. Keep the portions tall and a little craggy rather than flattening them. Arrange on a parchment-lined tray, cover, and refrigerate for at least 1 hour. For the deepest flavor and best texture, chill 12 to 24 hours.
8. Heat the oven: When ready to bake, preheat the oven to 350°F (175°C) with a rack in the center. Line cool baking sheets with parchment paper. Space the chilled dough portions at least 2 1/2 inches apart so the cookies can spread without touching.
9. Bake: Bake one sheet at a time for 11 to 13 minutes, until the edges are set and golden but the centers still look slightly underdone and soft. Do not wait for the centers to look fully baked; residual heat will finish setting them and preserve the chewiness.
10. Finish and cool: As soon as the pan comes out of the oven, press a few extra pieces of chocolate into the tops if desired and sprinkle very lightly with flaky sea salt. If a cookie spread unevenly, gently nudge it round with the edge of a large cutter or spoon while still hot. Let the cookies cool on the baking sheet for 8 to 10 minutes, then transfer to a wire rack.