1. Heat oven to 350 degrees. Butter three 8-inch cake pans; line bottoms with parchment. Dust pans with cocoa; tap out any excess. Set aside. Sift cocoa into a medium bowl. Whisk in 1 cup boiling water. Let cool.

2. In bowl of an electric mixer fitted with paddle attachment, cream butter on low speed until light, 3 to 4 minutes. Gradually beat in sugar until fluffy, 3 to 4 minutes, scraping down sides twice. Beat in vanilla. With mixer running, gradually add eggs, scraping down sides twice. Beat until smooth and fluffy.

3. In a large bowl, sift together flour, baking soda, and salt. Whisk milk into the cocoa mixture. With mixer on low speed, alternate adding the flour and cocoa mixtures, a little at a time, starting and ending with the flour mixture. Divide evenly among pans.

4. Bake until a cake tester inserted into the middle of each cake comes out clean, 35 to 40 minutes. Transfer cakes to a wire rack for 15 minutes. Invert pans, releasing cakes, and return to rack to cool, top sides up.

5. In the upper pot of a double boiler, combine chocolate and heavy cream. Place assembled double boiler over medium heat. Stir occasionally with a rubber spatula until chocolate has melted and mixture is smooth. Remove from heat; stir in cognac, if using.

6. Prepare an ice bath in a large bowl. Transfer 2 cups of chocolate glaze to a medium bowl. Set glaze over ice bath; stir until glaze reaches the consistency of thick frosting.

7. Place one cake layer on a cardboard cake round. Using a frosting spatula, spread 1/2 cup frosting on top. Set the second layer on top of the first; spread another 1/2 cup frosting on top. Stack the third cake layer on other two. Spread remaining cup of frosting on top and sides, creating a smooth surface.

8. Set a wire rack over a baking pan. Place cake on rack. Using a ladle, pour the remaining warm glaze over top and sides. Let stand until glaze has set, about 20 minutes. Just before serving, arrange the pansies on top.