Preheat the oven to 350°F. Line an 8x8-inch baking pan with parchment paper. Set aside.
Place the butter and chocolate chips in a heatproof glass bowl and microwave in 30-second intervals, stirring between each blast of heat, until smooth. Add the sugar and whisk well. Set aside to cool for about 5 minutes.
6 ounces dark chocolate chips, 6 tablespoons unsalted butter, 1 cup granulated sugar
Add the eggs to the cooled chocolate mixture and whisk well until the batter thickens.
2 large eggs
Add the coffee and vanilla and mix until combined, about 1-2 minutes.
2 tablespoons brewed black coffee, 2 teaspoons pure vanilla extract
Whisk in the flour, cocoa powder, baking powder, and salt until well combined.
¾ cup all-purpose flour, ¼ cup unsweetened cocoa powder, ½ teaspoon baking powder, ½ teaspoon kosher salt
Fold in nuts or chocolate chips, if using.
½ cup chocolate chips, ½ cup chopped nuts
Transfer the batter to the prepared baking pan.
Bake for 25-30 minutes until top is dry, then remove to cool completely.