Preheat the oven to 175°C (347°F). Grease an 8x8 inch tin and line it with parchment paper, leaving some overhang on the sides.
Dry roast the walnuts, almonds and cashews in a pan for 3 to 4 minutes until fragrant. Cool, then toss with the raisins, dates and 1 tablespoon of flour.
In a large bowl, whisk the melted butter and sugar for 1 to 2 minutes until combined.
Add the eggs one at a time, whisking well after each. Keep whisking for about a minute until the mixture looks glossy and slightly pale.
Stir in the melted dark chocolate until smooth.
Sift in the flour, cocoa powder, baking powder and salt. Fold gently until just combined.
Fold in the dry fruit mix, saving a spoonful for the top.
Pour the batter into the tin, smooth the top, and sprinkle the reserved dry fruits and a pinch of flaky salt on top.
Bake for 22 to 25 minutes, until a toothpick comes out with a few moist crumbs (not clean, not wet).
Cool completely in the tin for at least 1 hour. Lift out and cut into 12 squares with a warm, dry knife.