Preheat oven to 350°F (180°C). Grease and flour a baking sheet. In a large bowl, with an electric mixer on high, beat the butter and sugar until pale and creamy. Beat in the eggs, vanilla, and lemon zest. Add the flour, baking powder, and salt. Blend to combine. Stir in the nuts. Halve the dough and, with lightly oiled hands, roll each half into a log about 2 inches (5 cm) thick and 12 inches (30 cm) long. Place the logs on the baking sheet and bake in the centre of the oven for 25 minutes, or until golden.
Remove the baking sheet from the oven and place on a rack to cool. Transfer the logs to a bread board. Using a serrated knife, slice them about 3/4 inch (2 cm) thick at a 45? angle. To avoid crumbling, use firm, decisive strokes. Place the slices flat on a baking sheet and return to a 350°F (180°C) oven for about 10 minutes, turning once, to dry them. Cool on a rack. The cookies will keep in an airtight container for 2 to 3 weeks.