In large bowl with mixer at med speed, beat butter & sugar until light & fluffy; beat egg in well. At low speed beat in flour & baking powder until well blended. Flatten dough to disk shape; refrigerate, wrapped in plastic wrap, 1 hour until firm. Heat oven to 375. Work with half dough at a time; keep remainder refrigerated. Roll out to 1/8" thick between two sheets of wax paper or plastic wrap & cut into 2" squares. In small bowl stir in ricotta cheese, confectioners' sugar & lemon peel until smooth. Spoon 1 teaspoon ricotta mixture into center of half of dough squares; brush edges with beaten egg white. Top with remaining squares; using fork, press edges to seal. Brush with egg white; sprinkle with multicolored nonpareils. Bake 12-15 minutes. ----