To make cake:
Preheat oven to 350°. Butter two 8-by-2-inch square cake pans, line bottoms with parchment; butter and dust with cocoa.
In electric mixer bowl, sift cocoa, flour, sugar, baking soda, baking powder and salt. On low, stir in eggs, 1 1/2 cups warm water, buttermilk, vegetable oil and vanilla until smooth, about 3 minutes.
Divide batter between prepared pans. Bake, rotating once, until a tester comes out clean, 35 to 45 minutes. Let cakes cool on wire rack for 20 minutes, then remove from pans and cool completely, right side up on rack.
To make frosting:
Makes 16 cups of icing. In a small, heavy saucepan over medium heat, heat 2 cups sugar, the corn syrup and 2/3 cup water, stirring occasionally, until sugar has dissolved. Wash down sides of pan with pastry brush dipped in water to keep crystals from forming. Increase heat to high; bring to boil without stirring. Cook until mixture is 235°on a candy thermometer.
While syrup cooks, place egg whites in bowl of an electric mixer fitted with whisk attachment. Whip on medium speed until frothy. When syrup in pan has reached 235°, pour hot syrup into egg whites in a slow, steady stream down side of bowl. Increase mixer speed to medium-high; whip until fluffy and cool, 7 to 10 minutes. Add vanilla; whisk well.
Using a serrated knife, trim tops of both cake layers to be level. Brush away excess crumbs. Place bottom layer on 8-inch cardboard square. Tint icing blue with food coloring. Spread 1 1/2 to 2 cups icing over top of bottom layer. Top with second layer, cut side down. Spread 2 cups icing in over cake in a thin layer.
Place assembled cake on inverted cake pan to raise bottom edge for easier decorating. Use a large offset spatula to generously cover all of the cake with more icing. Use the long edge of the spatula to smooth the icing on each side.
Press cake comb into sides to make vertical lines around the cake, dividing each side into 4 equal parts; these guides will help you keep the waves even and neat. Pull cake-decorating comb or serrated knife across the sides, making a U shape from one line to next to form waves.
On top of cake, use small offset spatula to pull the still-moist icing straight up, creating frothy waves. Press decorated fish cookies
into sides of cake; icing will help them stick.