For the Cake:
1 box (18.25-ounces) chocolate cake mix
For the Filling:
5 tablespoons all-purpose flour
½ cup salted butter
1 cup granulated sugar
1¼ cups whole milk
½ cup vegetable shortening
For the Icing:
1 teaspoon vanilla extract
½ cup salted butter, melted and cooled to room temperature
2 tablespoons hot water
2 tablespoons whole milk
3 ounces semisweet chocolate, melted and cooled slightly
3 cups powdered sugar
1. Make the Cake: mix collectively the cake combine as directed on package deal and bake in a 10×15-inch jelly roll pan, greased and/or floured, as steered on the box. Cool to room temperature.
2. Make the Filling: in the meantime, mix the milk and flour in a small saucepan and cook dinner over low warmness, whisking continually, until the mixture thickens to the consistency of pudding, about 5 to eight minutes. Set aside and funky to room temperature.
3. Cream together the sugar, butter and shortening on medium pace until mild and fluffy, about 3 minutes. Add the cooled flour/milk combination and beat on high for 7 minutes. Spread on the cooled cake and refrigerate except set.
4. Make the Icing: In a significant bowl, mix together the entire icing ingredients on medium-excessive pace unless creamy and tender. Spread over the filling on the cake and serve. Duvet leftovers with plastic wrap and refrigerate.