Place the butter in a medium-sized saucepan set over medium heat. Allow it to melt completely, then continue cooking, swirling the pan often, until the butter foams and develops a golden brown hue with a nutty aroma—this should require about 3–4 minutes. Measure 95g of the browned butter into a mixing bowl and leave it to cool for 15–20 minutes to prevent scrambling the egg later. Once sufficiently cooled, whisk in the brown sugar and granulated sugar just until combined. Introduce the egg and whisk vigorously for 1–2 minutes, or use an electric mixer, until the mixture becomes paler and thickens. Blend in the vanilla extract thoroughly. Add the flour, salt, espresso powder if using, baking powder, and baking soda, then fold gently with a rubber spatula until faint flour streaks remain. Incorporate the chopped chocolate, folding to fully combine while finishing the flour integration. Line a quarter sheet pan or suitable container with parchment paper. Using a 2-tablespoon cookie scoop (approximately 55g per scoop), portion the dough and place the mounds close together on the parchment. Cover securely with plastic wrap or a lid and refrigerate for at least 30 minutes. About 10 minutes into chilling, preheat the oven to 350°F (180°C) and line two baking sheets with parchment. Arrange six dough balls on one prepared sheet. If preferred, gently flatten, press additional chocolate pieces on top, and roll back into balls, spacing them apart evenly. Bake for 11–13 minutes, until the edges are set and golden brown—bake closer to 11 minutes for a softer center or up to 13 for a firmer texture. Remove from the oven and, if desired, use a slightly larger cookie cutter to ‘scoot’ them into perfect rounds. Sprinkle with flaky sea salt if you wish. Allow the cookies to cool on the tray for 10–15 minutes before transferring to a wire rack. Repeat the process with the remaining dough. Store any leftover cookies in an airtight container at room temperature.