Preheat your oven to 350 degrees F. Lightly grease a 8x8 baking pan with butter. Lining your pan with parchment paper is optional but makes for easy removal.

In a microwave-safe bowl, combine the butter and chopped chocolate bar. Microwave the bowl on low power in 20-30 second intervals, stirring after each interval, until the mixture is melted and smooth. Remove the bowl from the microwave and gradually whisk in the sugar until well melted and incorporated.

Whisk in eggs and extra yolk, one at a time, making sure it is well combined before adding the next one. Whisk in the vanilla extract and coffee (if using).

Mix in the cocoa, salt, and flour, scraping the sides, until fully combined. Fold in chocolate chips. Rest the batter for 10 minutes.

Pour the batter into the prepared pan and gently smooth out the surface. Bake for 25 to 30 minutes, until a toothpick inserted into the center comes out just barely clean. Don’t overbake.

Lift brownies from pan. Remove the parchment paper and let cool on a wire rack.