Gluten-Free Chocolate-Tahini Brownies

Preheat oven to 350°F. Line an 8x8" baking dish with 2 overlapping pieces of parchment, leaving overhang. Whisk cornstarch and cocoa in a bowl until no lumps remain. Heat chocolate, oil, and 1 Tbsp. tahini in a small saucepan over low, stirring, until melted and smooth, about 1 minute; remove from heat.

Using an electric mixer on medium-high speed, beat eggs and both sugars in a large bowl until light, smooth, and doubled in volume, about 4 minutes. Reduce speed to medium and beat in salt and vanilla. Scrape in chocolate mixture and beat to incorporate; beat in cornstarch mixture. Increase speed to medium-high and mix until batter is thick and stiff, about 1 minute. Scrape into prepared baking dish and smooth top.

Stir agave nectar into remaining 3 Tbsp. tahini in a small bowl. Dollop spoonfuls over batter in 5 or 6 spots; drag a skewer or toothpick through to swirl.

Bake brownies until edges are puffed, top is browned, and a tester inserted into the center comes out with a few moist crumbs attached, 22–26 minutes. Let cool. Remove from pan; cut into squares.

Brownies can be baked 3 days ahead. Store tightly wrapped at room temperature.