Heat oven to 350°F. Grease and flour 13x9-inch baking pan; set aside.
Combine flour, baking powder and salt in bowl. Set aside.
Beat sugar and butter in another bowl at medium speed, scraping bowl occasionally, until creamy. Add 1 egg white at a time, beating well after each addition. Add vanilla; mix well. Add flour mixture alternately with milk, beating at low speed after each addition just until mixed.
Pour batter into prepared pan. Bake 30-35 minutes or until toothpick inserted near center comes out clean. Cool in pan on cooling racks 10 minutes. Loosen edge of cake by running knife around inside edge. Carefully remove cake from pan; cool completely.
Trim edges from cake; cut cake into 48 (1 1/2-inch square) pieces.
Combine sugar, cream of tartar and water in 3-quart saucepan. Cook over medium heat, stirring occasionally, 12-14 minutes or until mixture comes to a full boil. Cover; boil 3 minutes. Uncover; continue cooking 15-20 minutes or until candy thermometer reaches 228°F to 234°F, or small amount of mixture dropped into ice water forms a 2-inch soft thread. Remove from heat; cool to 110°F or 1 hour or until bottom of pan is slightly warm to touch. (Do not stir.) Stir in powdered sugar, 1 teaspoon almond flavoring and food color, if desired.
Place cooling rack over waxed paper. Place 1 cake piece on fork; drizzle icing over top and sides of cake, making sure each side is covered. Place onto rack; let stand until icing is set. (If icing becomes too thick, reheat over low heat 2-3 minutes or until thin consistency and easy to drizzle.)
Garnish each petit four with candy flowers or frosting flowers, if desired.