Heat oven to 350°F. Grease and flour 13x9-inch baking pan; set aside.
Combine flour, baking powder and salt in large bowl. Set aside.
Beat sugar and butter in large bowl at medium speed, scraping bowl occasionally, until creamy. Add 1 egg white at a time, beating well after each addition. Add vanilla. Reduce speed to low. Add flour mixture alternately with milk, beating well after each addition just until mixed.
Pour batter into prepared pan. Bake for 30 to 35 minutes or until toothpick inserted near center comes out clean. Cool on wire 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, until mixture comes to a full boil (12 to 14 minutes). Cover; boil 3 minutes. Uncover; continue cooking until candy thermometer reaches (228°F. to 234°F.) or small amount of mixture dropped into ice water forms a 2-inch soft thread (15 to 20 minutes). Remove from heat; cool to 110°F. or until bottom of pan is slightly warm to touch (do not stir) ( 1 hour). Stir in powdered sugar, 1 teaspoon almond extract and food color, if desired.
Place wire 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 wire rack; let stand until icing is set. (If icing becomes too thick, reheat over low heat until thin consistency and easy to drizzle (2 to 3 minutes)). Garnish each petit four with candy flowers or frosting flowers, if desired.