Melt the butter in a medium saucepan over medium heat, then let it cook for a further 3 to 4 minutes, stirring occasionally, until the milk solids have caramelized to a golden amber color and the butter smells nutty. Pour it into a large heatproof bowl, including all the caramelized solids at the bottom of the pan. Let it cool for 10 minutes. 170 g unsalted butter
Add the brown sugar, granulated sugar, espresso powder, vanilla, and egg to the butter. Beat until well combined. The espresso powder doesn't need to dissolve completely at this stage. 150 g soft brown sugar, 50 g granulated sugar, 1 large egg, 1 tsp vanilla extract, 2 1/2 tsp instant espresso powder
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Stir the flour mixture into the butter mixture until a dough just forms, then fold in the chocolate chips. If the cookie dough is still very warm from the butter, let it cool a bit more before adding the chips so they don't melt into the dough. 220 g all-purpose flour, ½ tsp baking powder, ½ tsp baking soda,½ tsp salt, 170 g semi sweet chocolate chips
Preheat the oven to 350°F (180°C). Line baking trays with parchment paper.
Roll the cookie dough into balls using about 2 tablespoons of dough per cookie, or a medium cookie scoop. If the dough feels greasy, let it stand for 5 minutes and it will firm up. Space the dough balls at least 2 inches apart on the prepared baking sheets. For thicker cookies, refrigerate the dough balls for 20 minutes before baking.
Bake for 10 to 11 minutes until the cookies have puffed and the edges look set. The centers will still be very soft. Don't overbake. The cookies will continue to firm up as they cool on the hot baking sheet.
While the cookies are still hot from the oven, sprinkle with flaky sea salt. Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack. Flaky salt to serve