Prepare the Cake:
Preheat oven to 350°F. Spray a 9-inch round springform pan with baking spray. Line the bottom with parchment paper and spray again.
In a heat-proof bowl set over a pan of simmering water, melt the butter and chocolate, stirring occasionally until smooth. Let cool for 5 minutes.
Whisk in cocoa powder, cassis, and vanilla extract. Set aside.
In a large mixing bowl, beat eggs, sugar, and salt on high speed for 3 to 5 minutes until pale yellow and tripled in volume.
Gently fold the chocolate mixture into the egg mixture using a rubber spatula until fully combined.
Pour the batter into the prepared pan and bake for 35-40 minutes until just set in the center.
Let cool in the pan for 30 minutes, then release the sides and invert onto a serving plate. Cool completely.
Prepare the Glaze:
In a heat-proof bowl set over simmering water, melt the chocolate and cream, stirring until smooth.
Remove from heat and whisk in cassis and vanilla. Let cool for 10 minutes, then spread evenly over the top of the cake.
Prepare the Berries:
15 minutes before serving, gently toss raspberries and strawberries with sugar and cassis liqueur.
Serve:
Slice the cake into wedges and serve with the macerated berries on the side.