Step 1

Preheat the oven to 325 degrees and prep a 9x13 sheet pan/dish with a swipe of shortening and dust of flour to prevent sticking (or you can use cooking spray, etc). Set aside.

Step 2

In a large bowl, whisk together all the ingredients except for the cake mix. Sift in the cake mix, then gently to combine (don't overmix). Spread the cake batter into the prepared dish, then bake for 25-27 minutes. Let the cake cool completely before decorating.

Step 3

While the cake is baking, make the filling. In a large bowl, whisk together the pudding mix and milk until it's thick. Refrigerate until the cake is cooled.

Step 4

In a glass dish, microwave the chocolate + heavy cream for 30 seconds and stir. Heat 30 seconds again and stir until smooth. Let cool for 10-15 minutes.

Step 5

In the bowl of a stand mixer fitted with a paddle attachment, whip up the butter until it's light and fluffy, then add in the cooled ganache, salt, vanilla, and cocoa powder until combined. Slowly add in the powdered sugar with the mixing speed on low, then increase the speed to the highest setting. Let the buttercream whip for 2 minutes until the texture and color lighten. *If you need to thin the texture, add in a 1/4 cup of heavy cream and mix again. Use a wooden spoon to beat out the air bubbles before adding the buttercream to your piping bag fitted with a wilton 1M piping tip.

Step 6

Spread on an even layer of the custard/pudding filling, then freeze for 5 minutes to set. Pipe on rosettes over the entire cake. There may be a little bit of buttercream left over, depending on how tight the rosettes are. If you don't want to pipe on buttercream swirls, use an offset icing spatula to spread on the buttercream in an even layer on top of the pudding layer. Serve within a couple days for max pudding freshness.