The yellow cake should be prepared as directed on the package. Evenly distribute the batter between two 9x13" pans (one greased with cooking spray, the other lined with parchment paper). Cakes should be baked for 15 minutes or until done, then totally cooled.

With a whisk, combine milk and flour in a pot. Stirring regularly, bring to a boil until thick. Turn off the heat and allow to cool.

Shortening, butter, sugar, salt, and vanilla should all be combined in a sizable basin (or stand mixer with a whisk attachment) and whisked until frothy.

Blend in the chilled milk mixture after adding it.

Place the cake that has been baked in a pan with parchment paper on a cooling rack. The parchment paper must be removed.

Spread the filling over the cake before adding the second layer.