Cream butter in a large mixing bowl; gradually add sugar, beating until light and fluffy. Add egg yolk (reserve egg white), almond paste and extract; beat until well blended. Stir in flour.
Spread mixture in a 33 x 23 x 5 cm 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 5 cm squares. Store in airtight containers.