Preheat the oven to 350°F, and grease a 10-inch skillet with butter or baking spray.
Add the chopped chocolate and unsalted butter to a microwave safe bowl, and melt. Stir to combine, then let cool briefly to avoid cooking and scrambling the egg. Add in milk, vanilla extract, eggs, and the egg yolk, and mix until combined and smooth.
2 sticks unsalted butter, 4 ounces dark chocolate chopped, 2 large eggs, 1 egg yolk, 2 tablespoons milk, 1 tablespoon vanilla extract
Add the brown sugar, powdered sugar, cocoa powder, espresso powder, flour, kosher salt, and optional chocolate chips or chunks and stir until smooth and there are no lumps.
½ cup + 2 tablespoons brown sugar, 1 cup powdered sugar, ⅓ cup Dutch processed cocoa powder, 1 teaspoon espresso powder, 100 grams flour, 1 teaspoon kosher salt, ½ cup dark chocolate chunks or chips
Add the batter to a greased 10-inch skillet and bake for 20-25 minutes, or 30-40 minutes if baking in an 8x8-pan, underbake slightly. Let cool for at least 20 minutes before cutting in and enjoy!