Preheat the oven to 180° Celsius.

Whisk the flour, ground hazelnuts, baking powder, salt, and cinnamon together in a large bowl.

In a separate bowl, whisk the butter and the sugar together with a mixer until the texture is smooth and the sugar dissolves. On medium speed, add the eggs one at a time. Beat in the bananas and the vanilla sugar until combined. 

With the mixer running on low speed, add the dry ingredients with the wet ingredients until combined. Add the chopped hazelnuts as a last step.

Spoon the batter into a prepared cake form (30 cm) and bake for 60 mins. Test with a toothpick if the middle of the cake is done. Remove from the oven when done.

In a small pan, heat up the jam until liquid. With a kitchen brush, coat the top of the hot cake with the jam, and allow it to cool on a wire rack. 

Store banana bread either for two days at room temperature, or up to a week in the refrigerator.