Place poppy seeds and milk in a small bowl; soak for 1 hour. Add vanilla. In a large bowl, cream butter and sugar until light and fluffy. Add poppy seed mixture.
Sift together the flour, baking powder and salt; add to creamed mixture. Fold in egg whites. Spread into two well-greased and lightly floured 8-in. round baking pans.
Bake at 375° for 20-25 minutes or until a toothpick comes out clean. Cool for 10 minutes before removing from pans to wire racks to cool completely.
For filling, in a small saucepan, combine the sugar and cornstarch. Stir in milk until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat; cook and stir 2 minutes longer. Remove from the heat. Stir a small amount of hot filling into egg yolks; return all to pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer. Remove from the heat. Gently stir in vanilla and nuts of desired.
Split cakes in half and spread filling between layers. Chill. Just before serving, dust with confectioners' sugar.