Cream butter in a large mixing bowl; gradually add sugar, beating until light and fluffy. Add egg yolk (reserve egg white), almond paste, and flavoring; beat until well blended. Stir in flour.
Spread mixture in a 13 x 9 x 2 inch baking pan. Beat egg white (at room temperature) until foamy; brush over entire surface of dough, and sprinkle with almonds. Bake for 35 minutes or until lightly browned. Cool thoroughly, and cut into 2 inch squares. Store in airtight containers.