Preheat the oven to 160°C. Grease two 20cm round cake pans and line base and sides with baking paper. Place chocolate, cocoa and 1/2 cup (125ml) water in a heatproof bowl set over a saucepan of simmering water (don't let the bowl touch the water) and stir until melted and smooth. Cool to room temperature, then whisk in eggs, sour cream and 1 tsp vanilla.
Place flour and sugars in a stand mixer fitted with the paddle attachment and beat until combined. Add oil, butter and 1 cup (250ml) chocolate mixture, and beat on low to combine. Add remaining chocolate mixture and beat to combine.
Divide between prepared pans and smooth surface with the back of a spoon. Bake for 1 hour 20 minutes to 1 hour 30 minutes or until a skewer inserted in the centre of a cake comes out clean. Set aside in pans to cool completely, then brush each cake with raspberry liqueur.
For the rhubarb jam, place rhubarb, sugar, vanilla pod and seeds and 2 tbs water in a saucepan over medium heat, stirring until sugar is dissolved. Cook, stirring occasionally, for 10 minutes or until rhubarb breaks down.
Meanwhile, soak gelatine in cold water for 5 minutes to soften. Squeeze excess water from gelatine, then add to rhubarb jam, stirring until melted and combined. Set aside to cool completely at room temperature.
To make the chocolate ganache, place chocolate and cream in a heatproof bowl set over a saucepan of simmering water (don’t let the bowl touch the water) and stir until melted and smooth. Remove from heat and set aside to cool to room temperature.
When ready to serve, whisk creme fraiche, cream and remaining 1 tsp vanilla together to stiff peaks. Place 1 cake on a cake stand and top with creme fraiche mixture, then spread over rhubarb jam. Top with second cake and spoon over chocolate ganache. Scatter with freezedried raspberries to serve.