Place the butter in a medium saucepan over medium heat. Allow it to melt completely, then continue cooking while swirling the pan frequently until the butter foams and turns a golden brown with a nutty aroma, which should take about 3 to 4 minutes. Measure out 95 grams of the browned butter (see Notes section below) into a medium or large mixing bowl and set it aside to cool for 15 to 20 minutes, ensuring it doesn’t scramble the eggs when added. Once the browned butter has cooled, add both the brown sugar and granulated sugar, whisking briefly to combine. Add the egg and whisk vigorously for 1 to 2 minutes—or use an electric mixer—until the mixture lightens in color and thickens. Stir in the vanilla extract thoroughly. In a separate bowl, combine the flour, salt, espresso powder (if using), baking powder, and baking soda. Fold these dry ingredients into the wet mixture with a rubber spatula just until some streaks of flour remain to avoid overworking the dough. Add the chopped chocolate and gently fold it in with the spatula to finish incorporating the flour. Line a quarter sheet pan or suitable container with parchment paper. Using a 2-tablespoon cookie scoop, portion out approximately 55 grams of dough per ball and place them on the parchment; they can be placed close together or touching. Cover the dough with plastic wrap or a lid and chill for at least 30 minutes. (Refer to the post’s “how to freeze cookie dough” section for freezing tips.) Ten minutes into chilling, preheat the oven to 350°F (180°C) and line two baking sheets with parchment paper. Arrange six dough balls on one sheet. If desired, gently flatten each ball, press extra chocolate on top, then roll back into a ball and space evenly on the pan. Bake for 11 to 13 minutes, or until the edges are set and golden brown—11 minutes will yield a gooey center, while 12 to 13 minutes will produce firmer cookies. Remove from the oven and, if you like, use a cookie cutter slightly larger than the cookies to gently round their edges. Sprinkle with flaky sea salt if using. Let the cookies cool on the pan for 10 to 15 minutes before transferring them to a wire rack to cool completely. Repeat the baking process with the remaining six dough balls. Store any leftovers in an airtight container at room temperature.