Preheat the oven to 325º Fahrenheit for dark pans or 350º Fahrenheit for light or glass pans. Grease 2 9” square cake pans with shortening, then dust with flour. (The only note I made here is that by putting shortening and flour up the sides of the pan, the cake did not cling to the pan and the sides slightly sunk in as it cooled, so possibly by greasing just the bottom only it would cling to the side better?) You can also line the pans with parchment paper to ensure they come out cleanly.
Mix cake mix, eggs, butter and milk in a large bowl using a mixer on medium speed. Scrape the bowl as needed. Beat for about two minutes.
Divide the batter evenly between the two prepared pans. Bake for 30 minutes or until the center of the cake springs back when you touch it.
Allow the cakes to cool in the pan for 5 minutes, then remove them to a wire rack to cool completely.
Make the frosting:
In a small saucepan over medium low heat, cook the flour and milk together, whisking constantly, until the mixture thickens to a pudding consistency.
Remove from the heat and allow the mixture to cool completely. (Move the mixture to another bowl and place it over a bowl of ice for it to cool more quickly.)
Using a mixer, cream the butter, shortening, salt and vanilla until smooth. Scrape the sides of the bowl as needed.
Add in the sugar and beat for 2 minutes.
Add in the cooled flour mixture. Use a mixer and beat until the mixture becomes fluffy like whipped cream. This may take a couple of minutes.
Assemble the cake:
If the cakes have domes, use a large knife to level them off so they are somewhat even.
Place one cake layer on a plate.
Spread all of the whipped frosting over the first cake layer.
Then place the second cake layer on top.
Dust with powdered sugar.
Slice and serve.
Store in the refrigerator.
The calories shown are based on the cake being cut into 12 pieces, with 1 serving being 1 slice of cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**