In a small bowl, combine the milk and the poppy seed. Let stand for 1 hour. Meanwhile, grease and flour two 9x1-1/2-inch round cake pans or line with waxed paper and coat with nonstick cooking spray. Set aside.
In a small bowl, combine flour, baking powder, and salt. Set aside.
In a large mixing bowl, beat butter with an electric mixer on medium to high speed for 30 seconds. Add the 1-1/2 cups sugar and the vanilla. Beat until well combined. Add dry ingredients and poppy seed mixture alternately to butter mixture, beating on low speed after each addition until just combined.
In a medium bowl, with a clean beater, beat the egg whites until stiff peaks form (peaks stand straight). Gently fold beaten egg whites into batter. Pour batter into prepared pans.
Bake in a 375 degree F oven for about 20 to 25 minutes or until a wooden toothpick inserted near the center of the layers comes out clean. Cool layers in pans on wire racks for 10 minutes. Remove the layers from pans. Let cool completely on wire racks.
Meanwhile, prepare the Custard Filling and chill in the refrigerator.
Split each cake layer in half horizontally. Place one bottom layer on a serving plate. Spread with 1/3 of the filling (about 1/2 cup). Repeat for two more layers. Place last cake layer on top.
Beat whipping cream and the 3/4 cup sugar together until mixture is spreadable. Spread over the sides and top of cake. Cover and chill up to 2 hours before serving. Garnish with sugared cranberries and geranium leaves, if you like. Makes 12 servings.
To make sugared cranberries, dip the cranberries into a mixture of water and meringue powder (sold at crafts stores and some supermarkets). Immediately sprinkle or dip into granulated sugar. Let coating dry.
In a heavy medium saucepan, stir together sugar and cornstarch. Stir in milk. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 2 minutes more. Remove from heat. Gradually stir 1 cup of the mixture into egg yolks. Add egg mixture to milk mixture in pan. Bring to a gentle boil. Cook and stir 2 minutes more. Remove from the heat; stir in vanilla. Transfer to a bowl. Cover the surface with plastic wrap and chill until completely cool, but don't stir.