Dry milk powder adds thousands more bits of browned nutty goodness to the butter in these cookies, an uber-rich chocolate chip cookie. The instant espresso powder adds a depth that plays well with the browned butter and chocolate. Don’t use regular coffee grounds since they won’t dissolve in the batter and would stay unpleasantly gritty.
Melt 1/2 cup (1 stick) butter in a medium saucepan over medium heat. Stir in the milk powder and cook, swirling and occasionally scraping the bottom of the pan with a heatproof rubber spatula or whisk, until the milk solids turn from creamy yellow to nutty golden brown, 3 to 5 minutes. Immediately scrape the contents of the saucepan into the bowl of a stand mixer or large bowl and let cool for 10 minutes, stirring occasionally.
Meanwhile, whisk together the flour, salt and baking soda in a small bowl; set aside.
Add both sugars to the browned butter in the bowl, then add the remaining 1/2 cup (1 stick) plus 2 tablespoons room-temperature butter. Attach the paddle to the mixer, or use a hand mixer, and beat on medium speed until lightened and fluffy, about 3 minutes. Add the eggs one at a time, beating for 30 seconds between each before adding the next. Beat in the espresso powder and vanilla bean paste. Add the dry ingredients and, using a rubber spatula, fold the mixture together just until no dry spots remain, then fold in the chocolate.
Using a 1-ounce cookie scoop or 2 tablespoons, portion out the dough and arrange the cookie balls on a parchment paper-lined baking sheet. Sprinkle a generous pinch of flaky sea salt on top of each. Cover the sheet with plastic wrap and refrigerate at least 6 hours or preferably overnight and up to 2 days.
Place racks in the upper and lower thirds of the oven, then heat the oven to 375 degrees. Line two baking sheets with parchment paper. Arrange 8 cookie balls on each baking sheet, spaced evenly apart. Bake the cookies, rotating the sheets from front to back and top to bottom halfway through cooking, until deep golden brown and firm around the edges, 10 to 11 minutes.
Transfer the baking sheets to racks and let the cookies cool completely on the baking sheets. Repeat baking the remaining cookies.
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