Special equipment: 9-inch cake pan
Preheat the oven to 375 degrees F. Butter the cake pan.
Toast the nuts and let them cool completely.
In a food processor, grind the nuts with the sugar until very fine.
Using a hand-held mixer, cream the butter; then add the sugar mixture. Add the eggs and mix. Add the sour cream and vanilla and stir. Mix in the banana puree. Stir in the flour, baking powder, and salt to make a smooth batter.
To bake, cover the bottom of the cake pan with some of the warmed hazelnut schmear (microwave 40 seconds or so). Fan some banana slices over it and spoon in the batter.
Bake until just set, about 35 to 40 minutes. Keep warm in a low oven and turn it out at the last minute.
Melt the butter. Remove from the heat and whisk in the brown sugar and the nuts. Store in the refrigerator.