In a large bowl, cream together the butter and sugar. Add eggs, vanilla and liqueur, and blend well. In separate bowl, combine the flour, salt and baking powder. Add the hazelnuts to the egg mixture, then add the flour and stir until just blended. On a floured surface, roll the dough into a cylinder 1 3/4 in diameter by 12. Place the dough on a cookie sheet and bake in 325ºF oven for about 25 minutes. Allow to cool for 5 minutes. Diagonally slice the roll into 1/2-inch cookies, and place face side up on the cookie sheet. Bake for 10 minutes. Allow to cool overnight.