Preheat oven to 350. Toast the walnuts in a baking pan for 5 minutes. Let cool and chop them coarse. Reduce the oven to 325. Cream the butter with the sugar in a large bowl. Beat in the eggs and mix well. Add the vanilla and Cognac. In another bowl, stir together the flour, baking powder, and salt. Add to the butter mixture with the chopped nuts. Stir just until combined. On a lightly floured surface, roll the dough into cylinders about 4 cm wide and 30 cm long. Place on a baking sheet about 6 cm apart and bake for about 25 minutes, until lightly browned on top. Remove from the oven and let cool for about 5 minutes.
Carefully remove the cylinders to a cutting board. Slice the cookies about 1 cm wide on the diagonal. Return them to the baking sheet, the cut surfaces down. Bake for 5-10 minutes, until the tops are lightly browned. Let cool and store in an airtight container.
NOTE: It is also delicious to add about 2 teaspoons of anise seed to the dough!