Preheat the oven to 160 ° C fan-assisted air. Grease the springform pan (22 cm).
Mix the butter and sugar until foamy and add the eggs one after the other and stir in well. Add flour, baking powder and cocoa and stir well. Then add the melted chocolate and sour cream and stir the mixture well.
Pour the dough into the mold and bake for about an hour (stick test!). Let cool in the mold.
For the glaze, heat the chopped chocolate and cream in a saucepan over low heat while stirring until the chocolate has melted. Let stand for 5 minutes and then brush the cake with it.