Step 1

Preheat oven to Prepare two, 9-inch baking pans by greasing them and lining them with parchment paper circles. Also at this point: Set out the two cups of unsalted butter for your buttercream on your counter to bring it to room temperature (pliable, but not soft) for making your frosting later.

Step 2

In large mixing bowl, beat sugar and eggs 3 minutes until fluffy and creamy. Blend in oil and vanilla and beat two minutes more. Combine boiling water and cocoa powder, stir to dissolve, mix in soda and salt. Pour into batter and incorporate. Add flour and mix until blended and smooth. Pour into prepared pans and tap pans to release bubbles. Bake at 350 for 35 to 40 minutes until cake springs back when touched. Remove from oven; let rest in pans for five minutes. Turn onto racks. Let cakes cool completely.

Step 3

While cake is baking, make whipped cream. Place 1 cup water in saucepan with 1/2 tsp agar agar. Heat to boiling; boil 4 1/2 minutes. Let solution cool just until you can immerse your finger in it–still quite warm and liquid–this takes about 3 to 3 1/2 minutes. While you’re waiting, combine 3 cups of cream with 1/2 cup powdered sugar and 1 tsp vanilla. Turn mixer to low speed. Before cream reaches soft peak stage, add 3 Tbsp of the warm liquid agar/water solution to cream all at once (there should be at least 2 1/2 to 3 tablespoons of this solution left after boiling) and whip until consistency firms up. Note: The whipped cream will not be super firm–just firmer than typical whipped cream.

Step 4

Make your light chocolate buttercream. Whip two cups of room temperature unsalted butter with 1/2 cup of vegetable shortening and 4 cups powdered sugar at low speed for 8 to 10 minutes until the mixture is fluffy. Add two tsp vanilla. Whip again just to incorporate. Mix together 2 1/2 Tbsp oil with 4 Tbsp Dutch process cocoa powder. Whisk into buttercream until evenly distributed.

Step 5

Assemble cake: Trim the “dome” off the top of each fudge layer to ensure each cake layer is exactly level and of the same thickness. (Note: if you are allergic to nuts, crumble this trim into crumbs and place on a parchment-lined baking sheet. Toast crumbs in the oven at 300 degrees until crispy. Crush these crumbs fine and reserve.) Place first fudge layer on a cake liner on a footed cake stand. Carefully pipe 1-inch of buttercream around the rim of the fudge layer, so you now have a standing lip of buttercream on the cake layer. Fill this with 1-inch tall whipped cream. Add several more spoons of whipped cream onto the center. Layer the second fudge layer of cake over the whipped cream layer. Using an offset spatula, carefully seal the outside edge of the cake (sealing whipped cream in) with buttercream, using a little more buttercream if needed. Frost top of cake with buttercream. Frost sides of cake with buttercream. Apply crushed nuts (toasted walnut and pecan) to sides of the cake. (Or, if allergic to nuts, apply the toasted cake crumbs to the sides of the cake.) Freeze the cake, which will ensure that the whipped cream layer and buttercream will firm up. Set cake out 1/2 hour to 45 minutes before serving to fasten.