Prep the pan: Heat oven to 350°F (175°C). Line an 8x8-inch metal pan with parchment, leaving overhang on two sides for easy lifting. Lightly grease the parchment.

Melt butter and chocolate: In a heatproof bowl set over a pot of simmering water, melt the butter and chopped chocolate, stirring until smooth. Remove from heat and let cool 5 minutes. You can also microwave in 20–30 second bursts, stirring between each.

Whisk in sugars: Add granulated and brown sugar to the warm chocolate mixture and whisk until glossy and combined.

Add eggs and vanilla: Whisk in eggs one at a time until fully incorporated. Add vanilla. Whisk for about 30–45 seconds more until the batter looks shiny and slightly thick—this helps that crinkly top.

Combine dry ingredients: In a separate bowl, whisk flour, cocoa powder, espresso powder, and salt to break up any lumps.

Fold gently: Use a spatula to fold the dry ingredients into the wet mixture just until no dry streaks remain. If using, fold in chocolate chips or chunks. Do not overmix.

Pour and level: Scrape the batter into the prepared pan and smooth the top. If you like a sweet-salty finish, sprinkle a pinch of flaky salt.

Bake: Bake 24–28 minutes, until the edges are set and the center wobbles slightly but doesn’t look wet. A toothpick inserted 2 inches from the edge should come out with moist, fudgy crumbs. The center may yield a little—that’s good.

Cool and set: Place the pan on a rack and cool at least 1 hour. For the cleanest slices and the gooiest texture, chill 30–45 minutes, then lift out and cut with a warm, clean knife.

Serve: Enjoy at room temp for maximum goo, or slightly warmed with ice cream for a classic sundae vibe.