Chop the chocolate, melt it on a hot water bath, let it cool down a bit. Grate the marzipan. Cream butter, 200 g sugar and salt. Stir in eggs one after the other. Mix flour, cornstarch, baking powder and cinnamon.
Alternate briefly with the milk. Pour 1/3 of the dough into another bowl. Stir in melted chocolate, plum jam and marzipan briefly. Fill both doughs spoon by spoon into a greased box form (approx. 24 cm long) sprinkled with breadcrumbs. Pull through spirally with a fork. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for about 1 hour. Take out and let rest in the form for about 20 minutes. Carefully turn out of the tin and let it cool down on a cake rack. Melt 20 g sugar in a pan until golden brown.
Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for about 1 hour. Take out and let rest in the form for about 20 minutes. Carefully turn out of the tin and let it cool down on a cake rack. Melt 20 g sugar in a pan until golden brown. Add cream and simmer for about 2 minutes. Mix the hazelnuts and spread them on the cake