Cake: Soak the POPPY SEEDS in the milk for 2 hours.
Preheat oven to 375°. Cream the butter until fluffy. Add the sugar and VANILLA and mix well. Add the poppy seed mixture. Gradually add the dry ingredients. Fold in the beaten egg whites. Pour into 2 greased 9-inch layer pans and bake at 375° for 25 minutes. Let cool in the pans for 5 minutes and then turn out onto wire racks to cool completely. Cut each layer in half. Spread some filling between each layer and then frost with Mimi's Never Fail White Frosting. Place in the fridge to firm up a bit before cutting. Mimi's cake tastes best when it's not completely cold, but the less cold the pudding is, the harder it is to slice.
Filling: Dissolve the cornstarch in a little of the milk in a medium saucepan. Add the rest of the milk and cook over medium-low heat until the mixture thickens, about 5-7 minutes, stirring constantly (a whisk works well for this). Remove from heat. Add the sugar to the beaten egg yolks. Gradually add the hot milk mixture to the yolks, stirring constantly. Return to the saucepan and cook over medium heat until thick and the mixture starts to bubble, stirring constantly, about 7-8 minutes. Once the mixture bubbles, cook for an additional minute. Remove from the heat and let cool. Stir in the VANILLA, cover by patting plastic wrap right on the surface, and refrigerate until ready to use.
Frosting: Beat the egg whites in a large bowl until peaks form and set aside. In a small heavy saucepan, combine the sugar, water, CREAM OF TARTAR and salt. Place on the stove over medium heat and stir until it starts to boil, 4-8 minutes. Cover tightly and cook over low heat for 2 minutes. Remove from the heat. While beating, pour the sugar mixture slowly over the egg whites. Beat until peaks form and the frosting is thick and glossy and has the consistency to spread. This process should take about 4-7 minutes. Add the VANILLA and beat just to combine. Frost the cake.
Prep. time: 40 minutes (plus 2 hours soaking time for softening poppy seeds)
Baking time: 25 minutes
Servings 16; Serving Size 1 piece (112g); Calories 310; Calories from fat 100; Total fat 11g; Cholesterol 105mg; Sodium 170mg; Carbohydrate 48g; Dietary Fiber 1g; Sugars 36g; Protein 5g.