Preheat oven to 350°F (175°C). Lightly coat a 17" x 11" jelly pan (1" lip) with non-stick baking spray, line with parchment paper, and spray the parchment. Set the prepared pan aside.

Make the chocolate layer: in a large bowl whisk together 2 cups all-purpose flour, 2 cups granulated sugar, ½ cup unsweetened cocoa powder, 1⅛ teaspoons baking powder and baking soda combined (1½ teaspoons each total), ⅛ teaspoon ground cinnamon, and ⅛ teaspoon salt. (If your baking powder and baking soda are separate, add 1½ teaspoons baking powder and 1½ teaspoons baking soda.)

Add ½ cup canola oil, 2 large eggs, 1 cup milk, and 2 tablespoons vanilla extract to the dry ingredients and mix until combined.

Carefully add 1 cup hot water to the batter and mix until incorporated. The batter will be thin; do not adjust. Pour the batter into the prepared jelly pan and spread evenly.

Bake the chocolate layer 20–25 minutes, or until a toothpick inserted in the center comes out clean. Cool 5 minutes in the pan, then invert onto a work surface, remove the parchment, and transfer to a cooling rack to cool completely. Trim edges if necessary.

When the chocolate layer is completely cool, use a long serrated knife to slice it horizontally into two even layers. Set both chocolate layers aside (wrap in plastic if you will refrigerate or freeze before assembling).

Reprepare the same jelly pan: spray, line with fresh parchment, and spray the parchment for the sponge layer.

Make the sponge cake: separate 6 large eggs into two bowls (whites in a large bowl, yolks in a smaller bowl). Add ½ cup of the divided granulated sugar to the egg whites and beat on medium-high until very stiff peaks form. Set the beaten egg whites aside.

Add the remaining ½ cup granulated sugar to the egg yolks and beat until the yolks are thick and pale yellow.

Gently fold the beaten egg yolks into the beaten egg whites until combined. Sift 1 cup cake flour, then add it to the egg mixture two tablespoons at a time, gently folding quickly and carefully after each addition until no dry streaks remain. Do not overmix.

Pour the sponge batter into the prepared jelly pan and bake 18–22 minutes, or until a toothpick inserted into the center comes out dry. Cool 5 minutes in the pan, invert onto a work surface, remove parchment, and transfer to a cooling rack to cool completely. Trim edges if necessary.

When the sponge cake is completely cool, slice it horizontally with a long serrated knife into two even layers. You should now have two chocolate layers and two sponge layers.

Make the peanut butter mousse: in a medium bowl beat 1½ cups heavy cream to medium–stiff peaks; set aside in the refrigerator.

In a large bowl beat together 2 cups creamy peanut butter and the 28 ounce packages cream cheese, softened, until smooth and well combined. Add 2 cups powdered sugar and 2 tablespoons vanilla extract and mix until incorporated.

Gently fold the whipped heavy cream into the peanut butter–cream cheese mixture until homogenous. Cover and refrigerate the mousse for 2 hours to firm.

Make the ganache: place 8 ounces semi-sweet chocolate broken into small pieces in a heatproof bowl. In a small saucepan over medium heat bring 1 cup heavy whipping cream and 1 tablespoon unsalted butter just to a simmer. Pour the hot cream mixture over the chocolate, let sit 1–2 minutes, then stir until smooth and glossy. Let the ganache cool 5–10 minutes until slightly thickened but still pourable.

Assemble the cake: place one chocolate layer on a cake stand or a piece of cardboard covered in foil. Spread an even layer of peanut butter mousse over it. Top with one sponge layer, then another layer of peanut butter mousse, then the second chocolate layer, and finally the second sponge layer so the cake alternates chocolate and sponge with mousse between them (ending with sponge).

Pour the ganache over the top of the assembled cake, letting it drip down the sides. Smooth as needed with an offset spatula.

Refrigerate the assembled cake at least 1 hour to set the ganache and mousse before slicing and serving. Store leftovers refrigerated.