Based on Maida Heatter's Budapest Coffee Cake from from her Book of Great Desserts with a few changes.

100g dark brown sugar
1tbsp cinnamon
1tbsp cocoa powder
35g sultanas
100g walnuts, toasted & chopped
60ml dark rum
140g butter
300g caster sugar
2 tsp vanilla extract
1 lemon, zest only
3 eggs, large
360g quark
390g flour
1½ tsp baking powder
1½ tsp baking soda
½ tsp salt
Preheat the oven to 375˚F/Gas 5/190˚C and butter a 22cm round kugelhof mould.
Toast the walnuts for about 10 minutes without burning them, then blitz in the food processor to medium coarse.  Mix all the filling ingredients and set aside.
Sift the flour, baking powder, baking soda and salt.
In a large bowl cream the butter, sugar, vanilla extract and the lemon zest until light and fluffy. Add the eggs one at a time, beating well after each addition.  Add the flour mix and quark in two additions alternating between them.
Spoon a thin layer of batter into the prepared mould. Top with half of the nut fillin, then a layer or batter and filling again, finishing with the batter layer on top.  Bake for 55 minutes.  Let it cool for about 10 minutes before turning out on a wire rack.  Once completely cooled transfer the cake to a serving plate.  You might need to slice off a thin layer from the bottom of the cake to make it stand upright.  Dust with icing sugar before serving.