For the cake, Preheat the oven to 350 degrees F. Spray a 9-inch round springform pan with baking spray. Line the bottom of the pan with parchment paper and spray it again with baking spray.
Melt the butter and chocolate together in a heat-proof bowl set over a pan of simmering water, stirring occasionally until smooth. Set aside to cool for 5 minutes. Whisk in the cocoa powder, cassis, and vanilla and set aside.
In the bowl of an electric mixer fitted with the whisk attachment (you can also use a hand mixer), beat the eggs, sugar, and salt on high speed for 3 to 5 minutes, until pale yellow and triple in volume. Pour the chocolate mixture into the egg mixture and carefully but thoroughly fold them together with a rubber spatula. Pour the batter into the prepared pan and bake for 35 to 40 minutes, until just barely set in the center. Allow to cool in the pan for 30 minutes and then release the sides of the pan. Invert the cake carefully onto a flat serving plate, remove the parchment paper, and cool completely.
For the glaze, melt the chocolate and cream together in a heat-proof bowl set over a pan of simmering water. Stir until smooth. Off the heat, whisk in the cassis and vanilla. Allow to cool for 10 minutes and spread over just the top of the cake.
Fifteen minutes before serving, toss the berries gently with the sugar and cassis. Cut the cake in wedges and serve with the berries on the side.