• Preheat the oven to 350 degrees. Line a baking sheets with parchment paper and set aside.
• In a small saucepan on medium/medium-low heat, add the 5 tablespoon flour and 1 cup milk together. Whisk these ingredients until combined, and continue whisking while it heats up, about 4-6 minutes. see notes
• Around minute 3 or 4, the mixture will start to resist and become very thick; continue stirring. When it starts to look like a paste/glue, remove from heat and stir to remove any remaining lumps. Set aside and allow to cool while you make your whoopie pies.

Make the pies

• In a large mixing bowl, add your dry ingredients- the cake mix, and additional flour. 
• Then add in the 4 eggs, ½ cup milk and ½ cup melted butter. 
• Using a wooden spoon or heavy-duty spatula, mix these ingredients until combined and no dry mix spots remain. 
• Using a 1 ½ tablespoon scoop or spoon, scoop out a whoopie pie onto the parchment paper lined baking sheet, leaving about 1 inch or more between pies. 
• Using the back of the scoop, gently push the batter around slightly to form the pie.
• Cook for 10-11 minutes, until the cakes are cooked through but not dry.
• Once cooked, remove from the oven and then transfer each pie to a cooling rack. Repeat until all the cookie cakes have been baked and let the cakes cool completely.

Make the filling

• In a large mixing bowl add the shortening and granulated sugar and using a hand mixer beat on medium speed until combined.
• Add the cooked flour mixture to the shortening /sugar mixture, and add in the 2 teaspoon vanilla extract.
• Whip on high speed for 8 minutes, or until light and doubled in size.  Making sure to scrape down the sides.
• Add your whipped filling into a piping bag or fitted with your favorite cake decorating tip o build the whoopie pies. *See notes

Build your whoopie pies

• Take your cooled whoopie pies and line the bottoms on a tray.
• Starting in the center, pipe about 1 to 1.5 tablespoon of filling, around the surface of the bottom piece.
• Take the top piece of the whoopie pie and place it on the filling, gently pressing the filling out to the edge of the cake.
• Repeat with your remaining cakes, filling on the bottom whoopie pie and topping with another cake.