Preheat oven to 350°F. Line a 9×13-inch pan with parchment paper, leaving overhang on the long sides.
In a bowl, toss cherries with sugar, cornstarch, lemon juice, and vanilla until evenly coated. Set aside while you make the brownie batter.
2 cups cherries, 1 tablespoon cornstarch, 1 teaspoon lemon juice, 1/2 teaspoon vanilla extract, 2 tablespoon granulated sugar
In a heatproof bowl, melt the butter and 10 ounces of chopped chocolate together (microwave in 30-second bursts or over a double boiler). Stir until smooth and glossy. Let cool slightly.
10 ounces dark chocolate, 1 cup unsalted butter
Whisk granulated sugar and brown sugar into the chocolate mixture.
1 1/2 cups granulated sugar, 1/2 cup light brown sugar
Add eggs one at a time, whisking well after each. Whisk in vanilla.
4 large eggs, 1 tablespoon vanilla extract
Sift together flour, cocoa powder, and salt.
1 cup all-purpose flour, 3/4 cup unsweetened cocoa powder, 1 teaspoon salt
Fold into the batter gently, just until no dry streaks remain. Fold in 1/2 cup chocolate chunks.
1/2 cup dark chocolate chunks
Spread about ⅔ of the brownie batter evenly into the prepared pan.
Spoon the cherry mixture evenly over the batter (avoid pooling liquid). Dollop the remaining brownie batter on top and gently swirl with a knife. Add some extra pitted cherries on top if desired.
Bake for 35 to 40 minutes, until the edges are set and the center looks just set. This will ensure fudgy brownies. If you’d like a cakier texture, bake it for 5 to 10 more minutes or until an inserted toothpick comes out clean.
Cool completely in the pan before slicing (this is key for clean cuts).