Place rack in centre of oven and preheat to 325ºF/150ºC.
Butter 9x13 in. (22x33 cm) pan and line with parchment paper. Set aside.
Beat eggs, maple syrup and butter together. Set aside.
In another bowl, mix flour, baking powder, zest and salt together. Add egg mixture, blending thoroughly. Stir in nuts.
Transfer batter to pan and bake until a toothpick inserted in the centre comes out clean, about 40 minutes.
Run a knife around the edges to loosen and turn out on a wooden board. Peel off paper and cool 25 minutes. Cut into slices 1/2 in. (2 cm) thick, then cut each slice in 3.
Set oven to 275 ºF/135 ºC.
Place on a cookie sheet and bake 60 minutes. Turn oven off and let sit 1 hour.