Preheat oven to 160ºC. Grease and line a 23 centimetre springform cake pan.
Place chocolate, butter, instant coffee and milk in heatproof bowl set over a pan of simmering water. Ensure that the bowl does not touch the water. Stir until mixture is silky smooth. Remove from heat and set aside to cool.
In a large bowl, sift flour and cocoa powder. Add Natvia. Fold in cooled chocolate mixture, followed by the eggs, vanilla extract and sour cream.
Pour batter into cake tin and cook for 60 minutes or until an inserted skewer comes out clean. Set aside and allow to cool completely in the cake tin.
To make the ganache, place all ingredients in a heatproof bowl. Either set over boiling water as above, or microwave at intervals of 30 seconds, stirring in between until mixture is smooth. Cool slightly to thicken up.
Once cake is cooled, spread with chocolate ganache and top with frozen berries. Serve immediately.