Preheat oven to 190 degrees C. Lightly grease baking tray.
Combine flour and baking powder; set aside. In a large bowl, cream together butter and sugar until light and fluffy. Beat in the eggs one at a time then stir in the lemon zest and almond essence. Stir in the flour mixture until smooth, then stir in the almonds.
Divide dough in two. Shape each portion into a loaf about 30cm long and 5cm wide. Place loaves onto the baking tray about 10cm apart and flatten slightly. Beat egg white until foamy and brush over tops of loaves.
Bake 20 to 25 minutes or until light brown. Cool on baking tray for about an hour.
Preheat oven to 170 degrees C. Cut baked loaves diagonally into 1cm thick slices. Lay slices on the baking tray.
Bake 10 minutes or until crisp. Cool on wire rack. Make several days before serving. Store in a paper bag to soften slightly. For longer storage, place in a sealed container.