How to Make It
Preheat oven to 425°. Prepare 9 8-inch disposable aluminum pans with cooking spray and rounds of parchment paper.
To prepare icing, in a large saucepan, whisk together evaporated milk, 3 cups sugar, cocoa powder, corn syrup and ¼ teaspoon salt over medium heat. Attach a candy thermometer to pot and cook, whisking occasionally, to just at soft ball stage (234-236°F), about 15 minutes.
Meanwhile, cream 1 ½ cups (3 sticks) butter and remaining 3 cups sugar using a large heavy duty stand mixer. Add in eggs, 2 at a time, until well incorporated, about 2 minutes. Add in 2 teaspoons vanilla. Sift together flour and baking powder in a medium bowl. Add remaining ¾ teaspoon salt to sifted flour mixture. Add flour mixture and milk alternately in 3 additions, beginning and ending with flour mixture. Scrape down sides of mixer bowl with a rubber spatula and mix to incorporate fully, about 1 minute.
Once chocolate icing reaches soft ball stage, reduce heat to simmer and add remaining cup (2 sticks) butter and remaining teaspoon vanilla. Stir to melt butter, about 2 minutes. Maintain a very low simmer until first round of cake layers are out of oven.
Using a kitchen scale, add 4 oz. batter to each prepared pan. Using an offset spatula, spread batter as evenly as possible on bottom of pans. Bake 4 layers at a time for 5 minutes in preheated oven. Remove from pan by placing a wire rack on top of pan and inverting pan to release cake layer. Remove parchment from cake layer and place first layer on cake stand. Remove icing from heat.
To assemble cake, pour about ¼ cup hot icing onto first cake layer, just enough to make a very thin layer of icing, but not enough to seep over sides of layer. Repeat with remaining 7 layers.
Prepare same cake pans with more cooking spray and rounds of parchment; weigh out 4 oz batter for each pan and spread layers as before. Bake 4 layers at a time for 5 minutes in preheated oven; continue layering cake with icing and baked cake layers. For top layer of icing, pour enough icing over top to seep over sides of cake. Use an offset spatula or small rubber spatula to spread a generous layer of icing to fully cover sides and top of cake. (Icing will be thicker at this point, and you will have some leftover). Let cake set at least 30 minutes or up to 1 day before slicing.