For the cake: Preheat the oven to 350 degrees F. Butter and flour 2 (8-inch) round cake pans. In a small bowl, pour 1 cup boiling water over the cocoa powder, whisking until smooth. Set aside. In a medium bowl, whisk together 2 1/4 cups flour, baking soda, and 1/4 teaspoon salt, and set aside. In a large bowl, combine the mayonnaise and sugar. Mix on medium speed until pale and fluffy. Add the vanilla and the cocoa mixture, mixing just until combined.
Gradually add the flour mixture, mixing gently just until combined. Divide the batter between the pans and bake until a toothpick comes out clean, about 25 minutes. Set the pans aside to cool completely. While the cakes cool prepare the filling.
For the filling: Whisk the sugar, cornstarch, 1/2 teaspoon salt and eggs together in a medium saucepan. Whisk in the milk until well combined. Continuously whisk over medium heat until the mixture comes to a boil. Boil and whisk for 1 to 2 minutes or until the mixture thickens. Remove the pan from the heat and add the chocolate chips, butter, and vanilla, stirring until combined and smooth. Pour into a bowl and cover with plastic wrap so that the wrap is touching the top of filling. Chill until ready to use.
For the frosting: In a large bowl, combine the butter and cocoa on medium speed until smooth. Add the vanilla. Alternate confectioner's sugar with heavy cream until the frosting reaches the desired consistency. Cook's Note: You may use more or less sugar and cream depending on when you reach the texture you prefer.
For the ganache: In a microwave-safe bowl, combine the chocolate chips and heavy cream. Microwave on high 1 minute, then remove and whisk together. Return to microwave for 1 more minute on high, remove and whisk. Follow this step until the mixture becomes glossy, smooth and easily spreadable. If too thick you may add slightly more cream.
To assemble the cake: Place one cake layer on the cake plate. Cover top with chilled pastry cream until about a 1/2-inch high. You will not likely use all of the pastry cream but fit in as much of it as you can without it running over. Place the second cake layer on top of the filling. Smooth the sides and refrigerate for a few minutes to help it set up. Cook's Note: To prevent the cake from slipping while refrigerated, insert long wooden skewers to hold the cake together then remove before frosting.
With a large ladle pour the ganache into the center top of the cake, moving the ladle in circular motions to spread over the top of the cake. Add more ganache and repeat the motion as needed until the top of the cake is covered and smooth. Clean any ganache that drips down the sides. Return to refrigerator for a few minutes to firm up the ganache.
Spread a thin layer of frosting around sides of cake only then go back over it adding more. Set aside any unused frosting for piping the bottom and top border. Do not frost over the ganache. Using a piping bag fitted with a large star or shell tip, pipe a border around the bottom of the cake and around the top edge of the cake to cover the line where frosting and ganache meet. Refrigerate for a few hours before serving.
Cook's Note: Contrasting chocolate tastes and textures combine for a palate pleasing chocolate rush!