18 T unsalted butter
4-1/2 c cake flour, sifted
2 T baking powder
¼ t salt
2-1/4 c sugar
1-1/2 c milk
1-1/2 T vanilla
7 egg whites
6 oz semisweet chocolate, chopped
Preheat oven to 350. butter 17x12x1 rimmed sheet pan. Line bottom with parchment, butter it and dust pan with flour.
Sift together dries into a medium bowl and set aside.
Beat the butter in the bowl of an electric mixer fitted with paddle until creamy. Add sugar to butter in steady stream and continue beating until light and fluffy. Add dries alternating with milk and vanilla. Do not overmix. Set batter aside.
Whisk egg whites to soft peaks, fold 1/3 of the egg whites into the batter until combined. Fold in remaining whites in 2 more batches. Pour batter into pan and smooth top with metal spatula.
Bake until done in middle, about 30 minutes. Let cool in pan on wire rack 15 minutes before unmolding. Loosen sides with small spatula or knife, and invert on to greased wire rack, then invert again so top is on top. (This prevents splitting.) Cool completely before assembling or wrapping airtight to freeze. Repeat steps wit second batch of ingredients for second layer.
To assemble, trim tops so they are flat. Place on layer of cake on serving platter. Set aside, covered, 1 c of the icing for coloring. Spread top of cake layer with ¼ inch layer of icing, and cover with second cake layer. Frost cake using remaining icing. Chill cake.
Place chocolate in heatproof bowl set over pan of barely simmering water stir, using a plastic spatula, until melted. Remove bowl from heat and set aside to cool until liquid but no longer warm., about 15 minutes. Pipe desired decorations with chocolate and set aside 10 minutes to set.