Preheat the oven to 350°F. Butter and lightly flour the sides of two 9-inch cake pans, knocking out the excess flour. Line the bottom of each pan with parchment paper cut to fit exactly.

To make the cake layers, sift together the flour, cornstarch, baking powder, baking soda, cocoa powder, and salt into a bowl and set aside. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium-high speed until light and creamy. Slowly add the sugar and continue to beat on the same speed until light in color and fluffy. Add the eggs one at a time, mixing well after each addition until incorporated before adding the next egg. Stop the mixer and scrape down the sides of the bowl with the rubber spatula. With the mixer on low speed, add the flour mixture in 3 equal batches alternately with the buttermilk in 2 batches, beginning and ending with the flour mixture. Stop the mixer, scrape down the sides of the bowl, and then mix again for another few seconds.

Divide the cake batter evenly between the prepared cake pans. Bake until the top springs back when lightly touched or a cake tester inserted in the center comes out clean, about 45 minutes. Let the cakes cool completely in the pans on a wire rack.

When the cakes are cool, turn them out by inverting the pans, and then turn the cakes upright. Using a serrated knife, slice off the domed portion from the top of each cake to make the tops flat. If you want to decorate the cake with crumbs as described in the headnote, reserve the slices for making the crumbs.

To make the crumbs, preheat the oven to 250°F. Line a baking sheet with parchment paper or a nonstick liner. Break up the cake slices and spread on the lined baking sheet. Place in the oven and toast until completely dry, about 1 hour. Let cool completely, then transfer to a food processor or blender and process until finely ground. Sift the crumbs through a medium-mesh sieve. (Don't use a sieve with fine mesh or the crumbs won't pass through.) Set aside.

To make the ganache, place the chocolate in a heatproof bowl, using the smaller amount of each ingredient if you will be decorating the cake with crumbs and the larger amount if you will not. Bring the cream to just under a boil in a small saucepan. Pour the cream over the chocolate. Let the mixture sit for a few minutes without stirring until the chocolate is partially melted, and then stir with a rubber spatula until smooth and shiny.

To assemble the cake, split each cake into 2 layers to make 4 layers in all. Transfer 1 layer to a serving plate. Using an offset spatula, spread 3 tablespoons of the caramel evenly over the cake layer. Spread a thin layer of ganache (about 1/4 inch thick) over the caramel. Top with a second cake layer, and again spread with 3 tablespoons caramel and then a thin layer of ganache. Repeat with the third cake layer, the remaining 3 tablespoons caramel, and a thin layer of ganache. Top with the fourth cake layer. Refrigerate the cake until the center seems firm, 1 to 2 hours. Cover the remaining ganache with plastic wrap and leave at room temperature for finishing the cake.

Remove the cake from the refrigerator. Using the rest of the ganache, frost the top and sides of the cake with the offset spatula. If you are going to coat the cake with the toasted cake crumbs, the cake must be evenly frosted and the ganache must be soft enough for the crumbs to adhere. If the ganache has hardened, use a kitchen torch to soften it slightly, or put the whole cake into a 400°F oven for io seconds or so, just until the chocolate looks shiny. Sprinkle the crumbs evenly over the top of the cake, then tilt and turn the cake so that they spill over the sides, adhering to them as they fall. If you are using only the ganache and not the crumbs, the ganache will be thicker on both the top and sides.

Serve the cake at room temperature. To store, cover tightly and keep in a cool place for up to 4 days. It is not necessary to keep this cake in the refrigerator.