In a small saucepan over medium heat, melt the butter, stirring frequently. It will foam and then the milk solids will start to turn golden brown. Keep stirring until you see deep amber flecks forming.

Remove the pan from heat as soon as the milk solids turn deep amber to prevent burning. Let the butter cool for 1 to 2 minutes until it's warm but no longer scorching hot.

Pour the warm browned butter into a large mixing bowl. Add the 1 cup of chopped dark chocolate and stir until smooth and fully melted. If the butter cooled too much to melt the chocolate, zap it in the microwave for a few seconds.

Add the brown sugar and granulated sugar to the bowl with the butter-chocolate mixture. Stir until well combined.

Mix in the sourdough discard, espresso powder, and vanilla extract, stirring until fully incorporated.Blend in the egg yolks, mixing until smooth.

Place a metal sifter over the bowl and sift in the flour, cocoa powder, baking powder, baking soda, and salt. Stir until just combined, ensuring no dry streaks remain.

Fold in the semi-sweet chocolate chips or chopped chocolate.

Using a #40 cookie scoop (about 2 tablespoons), scoop the dough into balls and place them close together on a baking sheet. (If you want large cookies, use ¼ cup scoops).

Cover with plastic wrap and refrigerate for at least 6 hours or up to 72 hours. (Chilling helps develop the flavor and improves texture.)

When ready to bake, preheat the oven to 350°F. Line a baking sheet with parchment paper.

Arrange the chilled dough balls 2 inches apart on the prepared baking sheet.

Bake for 8 to 9 minutes, or until the edges are set but the centers still look slightly soft. If using a ¼ cup scoop for larger cookies, bake for 11 to 13 minutes.