Cut 4 tablespoons (2 ounces/55g) of the butter into 4 pieces and add it to a large bowl.

Set a medium saucepan with a light-colored interior over medium heat and add the remaining 1½ sticks (6 ounces/170g) butter. Melt the butter, whisking occasionally and scraping the sides and bottom of the pot with a flexible spatula, until it foams and the milk solids turn a deep golden brown, 5 to 7 minutes.

Pour the hot brown butter over the cold butter in the bowl, making sure to scrape all the brown bits from the pan. Add the granulated and brown sugar and vigorously whisk for 90 seconds. (It’s okay if the mixture looks separated and oily.) Let the mixture sit for 5 minutes, stirring once or twice to help cool it.

Add the egg, vanilla, kosher salt, and espresso powder and whisk until emulsified. Whisk in the baking soda.

Add the flour and fold it into the mixture with a flexible spatula until only a few streaks of flour remain. Add the bittersweet and milk chocolate and fold again until just combined.

Scoop the dough into 2-inch mounds and place the mounds on a parchment-lined sheet pan that fits in your refridge. Cover the dough balls with plastic wrap and refrigerate for 8 to 36 hours.

When you are ready to bake, arrange a rack in the center of the oven and preheat the oven to 400°F. Line a second sheet pan with parchment paper.

Evenly space out the mounds of dough at least 1½ inches apart, 5 to each pan. Sprinkle with flaky sea salt (if using). Bake one pan at a time until the cookies are lightly browned on the edges but still quite soft in the center, 10 to 12 minutes. Remove the sheet pan, bang it on the counter once, and set the pan on a rack to cool. Repeat with the remaining pan. It’ll be tough, but try to let these cool for at least 10 minutes before eating.