1. Preheat the oven to 350°F (175°C) and line an 8×8-inch baking pan with parchment paper.

2. Melt the dark chocolate and unsalted butter together in a heatproof bowl over a double boiler or in short microwave bursts until smooth. Set aside to cool slightly.

3. In a large bowl, whisk together granulated sugar and brown sugar. Add the eggs one at a time, whisking until the mixture is thick, smooth, and shiny.

4. Stir in the sourdough discard and vanilla extract until fully combined.

5. Fold the cooled chocolate mixture into the wet ingredients with a rubber spatula until glossy and uniform.

6. Sift the all-purpose flour, cocoa powder, and salt into a separate bowl, then gently fold into the wet batter until no streaks remain.

7. Fold in the tart cherries, reserving a few to sprinkle on top. Pour the batter into the prepared pan and smooth the surface.

8. Bake for 28–32 minutes until a toothpick inserted in the center yields moist crumbs. Cool completely in the pan on a wire rack before lifting out and slicing into squares.