Toast walnuts in a 350 F oven for 4-6 minutes or until fragrant. Pulse in a food processor until coarsely ground. Combine nuts and flour. Pulse until finely ground. Beat eggs, maple syrup, sugar and salt for 5 minutes at high speed. Add espresso powder, vanilla, orange peel, cardamom, cinnamon and ginger and beat until mixture thickens and “ribbons” of batter fall from the beater. Fold in nut mixture. Pour into prepared pan and bake 30-40 minutes, or until a tester inserted into the center comes out clean. Place on rack to cool. While warm, spoon 4 tablespoons of syrup over the cake. Cool completely. Insert the tip of a knife between the cake and the pan side and loosen the cake from the pan; remove pan sides.