Coarsely chop the hazelnuts. In a bowl, combine the nuts, sugar, butter, liqueur and eggs; mix well. Stir in the flour, baking powder and salt.
Turn the dough out onto a floured surface and knead briefly; then form into a long loaf about 5cm wide. Place the loaf on a parchment-lined or lightly oiled baking tray. Bake in the middle of the oven for 25 minutes or until firm. The loaf will have a cake-like texture. Remove from the oven and cool.
Cut the loaf diagonally into 5mm to 1.25cm slices and lay out on a baking tray. Bake for 20 minutes more, turning them once until both sides are lightly browned and toasted. Cool and store in an airtight container.