Place the whole raw hazelnuts in an ovenproof dish and do the same for the hazelnut powder.
Bake at 170°C for a good 30 minutes. You'll need to stir the hazelnut powder fairly regularly to ensure even roasting. At the end, the powder takes on a lovely caramel color.
For whole hazelnuts, the skin should crack and they should be nicely browned. Pour the roasted hazelnut powder, brown sugar, flour, baking powder and salt into a bowl. Mix.
Pour in the 180g of egg white (to do this, collect the whites of 5 or more eggs, and weigh out the quantity required). Mix again. The dough should be homogeneous.
Pour into a mould lined with greaseproof paper and grease the inside edge. Pour in the dough, then arrange pieces of crushed toasted hazelnuts on top.
Bake for 40 minutes at 155°C over static heat (not convection heat), then lower the temperature to 120°C with steam function if you have this option, and bake for a further 30 minutes. Without the steam function, lower the temperature to 120°C by placing a dish of boiling water in the bottom of the oven and bake for 30 minutes. Leave the cake to cool, turn out carefully and sprinkle with powdered sugar as required.