Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan. In a saucepan, melt the butter over low heat. Remove from heat and stir in the granulated sugar, brown sugar, eggs, and vanilla. Mix in the flour, cocoa powder, and salt until just combined. Pour into the prepared pan and bake for 20-25 minutes. Let cool completely.

Chocolate Mousse Layer

Make the Mousse

In a small bowl, sprinkle gelatin over water and let it bloom for 5 minutes. In a heatproof bowl, melt the dark chocolate over a double boiler. Remove from heat and whisk in the gelatin mixture until smooth. In a separate bowl, whip the heavy cream with powdered sugar and vanilla until soft peaks form. Gently fold the whipped cream into the chocolate mixture until fully incorporated. Pour over the cooled brownie layer and refrigerate until set, about 2 hours.

Chocolate Ganache

Prepare the Ganache

Heat the heavy cream in a saucepan until just simmering. Pour over the chopped dark chocolate in a bowl and let sit for 5 minutes. Stir until smooth. Allow to cool slightly before pouring over the mousse layer. Refrigerate until set.

Berry Purée

Make the Berry Purée

In a blender, combine the mixed berries, sugar, and lemon juice. Blend until smooth. Strain through a fine mesh sieve if desired for a smoother consistency.