Preheat your oven to 350°F (180°C). Line an 8x4 inch loaf pan with parchment paper, leaving some overhang on the sides for easy lifting.
Add the butter and chopped chocolate to a microwave-safe bowl. Microwave in 30-second increments, stirring after each, until fully melted and smooth. 57 g unsalted butter, 42 g semi-sweet chocolate
While the chocolate mixture is still hot, stir in both sugars until well combined. 65 g granulated sugar, 50 g soft brown sugar
Mix in the egg and whisk by hand for a minute, then add vanilla, cocoa powder and salt until fully incorporated.1 large egg, 1 teaspoon vanilla paste or extract, 14 g unsweetened cocoa powder, ¼ teaspoon salt
Fold in the flour and chocolate chips. The batter will be thick and glossy. 32 g all-purpose flour, 85 g chocolate chips
Spread the batter evenly into the prepared pan and top with more chocolate chips. Bake for 26-30 minutes, or until a skewer inserted into the center comes out with a few moist crumbs attached but no wet batter or wobble to the middle.
Leave the brownies in the pan to cool completely on a wire rack before lifting them out and slicing. For the cleanest cuts, chill in the fridge for an hour once cooled, then slice with a sharp knife, wiping it clean between cuts.
Notes
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Use a good quality semi-sweet chocolate with real cocoa butter, not compound chocolate made with vegetable oil.
The brownies are done when a skewer comes out with a few moist crumbs, not wet batter, and not clean. Because this recipe uses so little flour and no leavening, these are very hard to overbake into something cakey. If you do leave them in too long, they'll firm up and dry out slightly rather than puff up, so the bigger risk is underbaking. If your brownies sink in the middle as they cool, give them a few extra minutes next time.
Store in an airtight container at room temperature for up to 5 days, in the fridge for up to a week, or freeze for up to 3 months.