In a large bowl, use an electric mixer to beat the butter on medium to high speed for 30 seconds. Add the sugar, lemon zest, vanilla and salt; beat until combined, scraping the sides of the bowl occasionally. Beat in as much flour as you can with the mixer. When it becomes too thick, stir in any remaining flour, lemon zest, and 1/3 cup of the almonds with a wooden spoon.
Press the dough evenly into the bottom of an ungreased 13x9x2-inch rimmed baking sheet. Sprinkle the remaining almonds over the top; press lightly into the surface.
Bake for about 20 minutes or until the top is golden. Cool in the pan on a wire rack. If desired, sprinkle with powdered sugar. Break or cut into pieces.
To freeze, break the cooled cookie into pieces. Place in a freezer- safe container, and freeze for up to 1 month. To serve, thaw in the container at room temperature for 15 minutes. If desired, sprinkle with powdered sugar.