1.

Cakes: Preheat oven to 350 degrees. Butter four 6-inch round cake pans. Line with parchment, and butter parchment; and dust with cocoa, tapping out excess.

2.

Stir cocoa and water in a small bowl until a smooth paste forms; let cool. Whisk together flour, baking soda, and salt in a medium bowl.

3.

With an electric mixer on medium speed, beat butter and sugar until pale and fluffy, 3 to 5 minutes. Mix in vanilla. Add eggs in a slow stream, mixing until incorporated and batter is no longer slick.

4.

Gradually whisk milk into cocoa paste. With mixer on low speed, gradually add both flour and cocoa mixtures to the butter mixture, and mix until combined.

5.

Divide batter among prepared pans. Bake until a cake tester comes out clean, about 30 minutes. Transfer pans to wire racks to cool 10 minutes. Turn out cakes onto racks to cool completely.

6.

Whipped Ganache Frosting: Meanwhile, put chocolate into a large heatproof bowl. Bring cream just to a boil in a saucepan. Pour over chocolate; add salt. Let stand 10 minutes. Whisk until chocolate is melted and mixture is smooth and shiny, scraping bottom of bowl with a flexible spatula. Let cool to room temperature, stirring often, until thickened, 45 to 60 minutes. With an electric mixer on medium-high speed, beat until paler in color and fluffy, 2 to 4 minutes. (You should have about 4 cups.)

7.

Chocolate Ganache Glaze: Put chocolate into a large heatproof bowl. Bring cream and corn syrup just to a boil in a saucepan, stirring to combine. Pour over chocolate. Let stand 10 minutes. Whisk until smooth and shiny, scraping bottom of bowl with a flexible spatula. (You should have about 3 cups.)

8.

With a serrated knife, trim tops of cake layers to level. Using an offset spatula, spread about 1 cup whipped ganache frosting on each of 2 cake layers; top with remaining layers. Spread remaining frosting over cakes.

9. Place 1 cake on a wire rack set over a parchment-lined rimmed baking sheet. Using a ladle, pour half of ganache glaze over top and sides of cakes, smoothing sides with a large offset spatula. Repeat with second cake. Let stand until glaze has set, about 20 minutes. Just before serving, arrange pansies on top of cakes.