Heat oven to 350°F. Lightly grease an 8-inch square pan and line with parchment, leaving overhang.
Stir melted butter, granulated sugar, and brown sugar in a bowl until glossy and combined.
Whisk in liquid egg and vanilla until the mixture looks thick and shiny.
Combine flour, cocoa, salt, and espresso powder. Sift or whisk, then fold into the wet mix just until no dry spots remain.
Fold in about half of the chocolate chips. Spread the batter evenly in the prepared pan.
Nestle dark chocolate chunks into nine evenly spaced spots. Lightly cover with a smear of batter. Sprinkle on remaining chips.
Bake 22–26 minutes until edges are set and centers are slightly soft. A toothpick should pick up moist crumbs, not wet batter.
Cool in the pan 10–15 minutes. Lift out, slice into 9 squares, and serve warm for a molten center.