Heat a conventional oven to 350°F (175°C), with a rack in the center. Line an 8-inch (20 cm) square metal pan with parchment, leaving an overhang on two opposite sides. Do not substitute a larger pan without expecting a thinner slab and different baking time.

Melt butter in a microwave-safe bowl in short bursts, stirring between them, until just melted and warm rather than sizzling. Whisk in sugar for about 1 minute, then cocoa until evenly combined. Let the mixture cool if it feels hot; it should be warm, not hot enough to cook the eggs.

Whisk in eggs one at a time, then vanilla and salt, until the mixture is smooth and slightly glossy. Add flour and fold with a spatula only until no dry streaks remain, scraping the bottom of the bowl. Fold in dark chocolate chips. Do not eat or taste the raw batter.

Spread the batter evenly in the prepared pan. Bake about 25–30 minutes, checking toward the end; ovens and pans vary, so allow longer if necessary. The edges and center should be set, and a toothpick inserted in the center should show moist crumbs but no liquid raw batter. If the tester hits a chocolate chip, check a nearby spot. Do not use a shiny top alone as proof of doneness.

Set the pan on a wire rack and cool completely, at least 90 minutes or longer if still warm. Lift out with the parchment sling. Cut into a 4 × 4 grid for 16 small squares, wiping the knife between cuts. Optional ice cream, nuts or frosting are excluded from the estimates.

Keep cooled plain brownies airtight in a cool room for up to 3 days, or wrap individual squares and freeze for up to 1 month for quality. Thaw wrapped portions before serving. Add perishable toppings only when serving.