PREPARE THE SOUR CHERRIES (IF USING KIRSCH):
If using kirsch, place the chopped dried cherries in a small bowl and pour the kirsch over them. Let soak for at least 30 minutes, or up to overnight. The cherries will plump and absorb the cherry brandy flavor. Drain any excess liquid before adding to the brownie batter.

MAKE THE CHOCOLATE BROWNIE:
Preheat oven to 350°F (180°C / 160°C fan). Line an 8x8 or 9x9 inch baking pan with parchment paper, leaving overhang on two sides for easy removal. Lightly grease the parchment.

In a double boiler or microwave-safe bowl, combine the chopped dark chocolate and butter. Heat gently, stirring frequently, until completely smooth and glossy. If using microwave, heat in 30-second intervals, stirring between each. Set aside to cool for 5 minutes.

In a large bowl, whisk together the granulated sugar, brown sugar, and eggs until well combined and slightly thickened, about 2 minutes. The mixture should be pale and ribbon-like. Whisk in the vanilla extract.

Pour the cooled chocolate mixture into the egg mixture, stirring gently until completely combined. The batter will be thick and glossy.

Sift the flour, cocoa powder, and salt over the chocolate mixture. Fold gently with a spatula until just combined—do not overmix, as this develops gluten and creates a cakey rather than fudgy texture.

Fold in the chocolate chips and drained sour cherries until evenly distributed.

Pour batter into the prepared pan and spread evenly. Bake for 25-30 minutes. Critical for texture: The brownies are done when the top has a thin, crackly crust and a toothpick inserted in the center comes out with moist crumbs (not wet batter, not clean). The edges should be set but the center should still have a slight wobble.

Let the brownies cool in the pan on a wire rack for at least 1 hour before cutting. For cleaner cuts, refrigerate for 30 minutes before slicing.

MAKE THE BLACKBERRY GEL:
In a medium saucepan, combine the blackberries, sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the berries break down completely and become jammy, about 10 minutes.

Press the mixture through a fine-mesh sieve into a clean bowl, extracting as much liquid as possible. Discard the seeds. You should have about ¾ cup of smooth purée.

Return the strained purée to the saucepan. In a small bowl, whisk the agar agar powder into the water until dissolved. Add to the purée and bring to a boil over medium heat, whisking constantly. Boil for 1-2 minutes to activate the agar.

Pour into a shallow container and refrigerate until fully set, at least 2 hours. Once set, transfer to a blender and blend until completely smooth and gel-like.

MAKE THE TOASTED HAZELNUT CREAM:
Preheat oven to 350°F (180°C). Spread hazelnuts on a baking sheet and toast for 8-10 minutes until golden and fragrant. Watch carefully—they can burn quickly. Cool completely.

Finely chop about three-quarters of the hazelnuts. Leave the remaining quarter coarsely chopped for texture contrast.

In a cold bowl, combine the heavy cream, powdered sugar, vanilla, and Frangelico if using. Whip with an electric mixer until soft peaks form. Gently fold in the finely chopped hazelnuts until evenly distributed. Keep refrigerated.

MAKE THE SOURDOUGH CRISPS:
Using a mandoline or very sharp knife, slice sourdough bread as thinly as possible—aim for 1-2mm thickness.

Preheat oven to 325°F (165°C). Brush both sides of each bread slice lightly with melted butter. Sprinkle one side with sugar and a tiny pinch of flaky salt.

Place sugar-side up on a parchment-lined baking sheet. Bake for 10-15 minutes, watching carefully, until golden and completely crisp.

ASSEMBLE AND SERVE:
Cut brownies into 8 equal portions using a sharp knife dipped in hot water. Place a brownie square slightly off-center on each plate. Add a generous quenelle of hazelnut cream alongside. Drizzle or dot blackberry gel around the plate.

Lean a sourdough crisp against the brownie. Scatter reserved coarsely chopped hazelnuts over the cream. Garnish with fresh blackberries, chocolate shavings, and mint if desired. Serve immediately.