In a large bowl, cream the butter, shortening and sugar until light and fluffy. Beat in vanilla. Combine the flour, baking powder and salt; add to creamed mixture alternately with milk, beating well after each addition. In a small bowl, beat egg whites until soft peaks form; gently fold into batter.
Pour into a greased 9-in. square baking pan. Bake at 350° for 20-25 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack to cool completely.
Cut a thin slice off each side of cake. Cut cake into 1-1/4-in. squares. Place 1/2 in. apart on a rack in a 15-in. x 10-in. x 1-in. pan.
In a large bowl, combine glaze ingredients. Beat on low speed just until blended; beat on high until smooth. Apply glaze evenly over tops and sides of cake squares, allowing excess to drip off. Let dry. Repeat if necessary to thoroughly coat squares. Let dry completely.
For frosting, in a small bowl, cream the butter, shortening and vanilla. Beat in confectioners' sugar and enough milk to achieve desired consistency. Place 1/2 cup each in two bowls; tint one pink and one green.
Cut a small hole in the corner of a pastry or plastic bag; insert #104 tip. Fill with pink frosting; pipe a rosebud on each petit four. Insert #3 round tip into another pastry or plastic bag; fill with green frosting. Pipe a leaf under each rose. Yield: 2-1/2 dozen (3 cups frosting).