Preheat oven to 350 degree F. Grease a 15x10x1-inch baking pan; line bottom of pan with waxed paper or parchment paper and grease paper. Set aside.
In a very large mixing bowl beat butter with an electric mixer on medium speed for 30 seconds. Beat in sugar until combined. Beat in egg yolks. In a medium bowl stir together graham cracker crumbs, flour, and baking powder; stir in milk and vanilla. Beat into butter mixture, about one-third at a time, beating on low speed after each addition just until combined. Stir in walnuts; set aside.
Wash beaters thoroughly. In a large bowl beat egg whites with an electric mixer on medium speed until stiff peaks form (peaks stand straight). Gently stir about half of the egg whites into graham cracker mixture to lighten; fold in remaining egg whites (batter may appear slightly curdled). Spread batter into the prepared pan. Bake for 25 to 30 minutes or until golden and top springs back when lightly touched. Cool in pan 10 minutes. Invert onto large cooling rack; gently peel paper off cake. Cool completely.
Prepare White Satin Frosting. Cut cake crosswise into 3 equal parts. Place one piece on serving plate; frost top with one-third of the frosting. Repeat with remaining cake and frosting. Garnish with crystallized ginger and walnuts, if desired. Serve at once or cover and store in refrigerator up to 8 hours. Makes 10 servings.
White Satin Frosting
In a small saucepan whisk together milk and flour until smooth. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 1 minute more. Transfer to a small bowl. Cover surface with plastic wrap; chill at least 2 hours or until very cold. In a large mixing bowl beat butter with an electric mixer on medium speed for 30 seconds; beat in 1-1/2 cups sifted powdered sugar and vanilla until fluffy. Beat in cooled mixture. Gradually beat in 2 to 2-1/2 cups additional sifted powdered sugar to desired spreading consistency. Cover and chill for 30 minutes before frosting cake. Makes 3 cups frosting.