- In a small mixing bowl, combine the cocoa powder and hot water. Stir until smooth, and then blend in the cold water. Add vanilla; stir until thoroughly blended and smooth. Set aside.
- In a medium bowl, combine flour, baking powder, baking soda, and salt; sift or whisk together to mix. Set aside.
- In a large bowl of an electric mixer, cream the butter, granulated sugar, and brown sugar until light and fluffy. Tip: To cream, start by placing the butter in the bowl, with an electric mixer on medium speed begin by beating the butter about 1 minute until it is smooth and light in color. With the mixer still on medium speed, slowly add the granulated sugar to the butter, either one tablespoon at a time, or in a very slow steady stream, taking from 4 to 5 minutes to add all of the sugar, then add the brown sugar, taking an additional 3 to 4 minutes, beating until the butter and sugar are fully incorporated and the mixture is a light, or pale yellow color, with a fluffy texture. While adding the sugars, stop the mixer occasionally to scrape the mixture off the paddle and scrape down the sides and bottom of the bowl with a rubber spatula so the mixture blends evenly.
- Add eggs one at a time, beating until thoroughly mixed. Tip: For each egg, crack the egg into a small bowl and whisk with a fork to thoroughly break up the egg before adding to the creamed mixture. Start with the mixer on low speed so the liquid from the egg doesn’t splatter, once the egg is partially mixed increase the speed to medium. Each egg should be fully incorporated into the mixture before adding the next egg, taking about one minute to blend in each egg.
- With the mixer on low speed, add about one third of the flour mixture, mix just until the flour is almost completely blended. Scrape the bowl down, and add about one half of the cocoa mixture, blending just until mixed. Scrape the bowl down again and continue alternating with the flour mixture and cocoa mixture, ending with the last portion of the flour, and stirring just until blended.
- Bake: Spoon the batter into the prepared pans and smooth the surface with the back of a large spoon. Bake 25 to 30 minutes or until a long toothpick, wooden skewer, or cake tester inserted in the center comes out clean. Remove from oven and place pans on a wire cooling rack to cool for 10 to 15 minutes then remove cake from the pans and place the cake on the wire cooling rack to finish cooling.
- Using a long-serrated kitchen knife, split each cake into 2 horizontal layers.
- Place one of the bottom halves of the layers on a cake plate. Using an offset spatula spread about 1 cup filling over the layer, spreading the filling out to the edges. Repeat with the next four layers, pressing down a bit on the cake before adding the filling to make sure everything is level. Place the last cake layer on top, upside down so the flat bottom of the cake is facing up to make a nice flat top. Spread remainder of filling over top of cake. The sides of the cake should show for the naked cake look.
- Decoration (optional:) Decorate with the cookie halves and mini-marshmallows.
- Refrigerate cake until ready to serve. Refrigerate leftovers.
