Heat oven to 350°F. Cream butter and sugar together till fluffy. Add eggs one at a time. Stir in vanilla. Sift flour, baking powder and salt together and add to creamed mixture. Stir in pecans. Divide dough into three equal pieces. Shape each piece into 2 1/2 inch diameter log. Place logs on a parchment lined baking sheet and press down to form the classic arched biscotti shape. Bake for 15 minutes. Remove to cutting board very carefully with thin spatula. Cut each log into 3/4 inch slices. Lay slices on their sides and return biscotti to oven to dry for 15 more minutes, turning over halfway through baking time. Cool on rack.