Turn on the oven at 360°F. Line a 9 inch round mold with parchment paper and set aside.
Blend hazelnuts and brown sugar to obtain a coarse flour, then put it in a bowl along with spelt flours, baking powder and salt. Mix well with a whisk.
Add the water little by little, stirring constantly with the whisk, until you get a smooth batter.
Pour the hazelnut cake batter into the mold and bake for 40 minutes, check with a toothpick and take the cake out of the oven if it comes out clean.
Let the cake cool down completely then cut it into two equal disks. Spread the lower with raspberry jam, top with the other disk and sprinkle with icing sugar, then enjoy.