• Preheat the oven to 350 degrees F. Line some baking sheets with parchment paper and set aside.
• Pro Tip: If you place the butter in a small saucepan on your stove under the oven vent, it will melt the butter evenly but not get it too hot. Room temperature butter will melt faster.

Prep the Filling Thickener:

• In a small saucepan on medium or medium-low heat, add the 5 tablespoons flour and 1 cup milk together. Whisk these ingredients until combined, and continue whisking while it heats up about 5 minutes.
• Note: Do not let this mixture come to a boil; keeping the heat at low/medium-low will result in the best paste texture. Continuous stirring to keep this mixture smooth is recommended.
• Around minute 3 or 4, the mixture will start to resist and become very thick; continue stirring. When it looks like a paste/glue, remove it from heat and stir to remove any remaining lumps. Set aside and allow to cool while you make your whoopie pies.

Make the Cake Pies:

• Line your baking trays with parchment paper, and set aside.
• Add your dry ingredients - the cake mix and additional flour- to a large mixing bowl.
• Then add the liquid ingredients to the dry ingredients: 3 egg whites, 1 egg, ½ cup whole milk, and ½ cup melted butter.
• Mix these ingredients with a wooden spoon or heavy-duty spatula until combined and no dry mix spots remain.
• Note: The batter should be thicker than a standard cake batter, about the consistency of a thin brownie mix, so it holds its shape when scooping out for baking but allows some spreading to occur during baking.
• Using a 1½ tablespoon scoop (about 1+ ounces), 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.
• This action will help create a round whoopie pie that's not too domed on top, although doming may occur due to variances in cake mixes.
• Note: Make sure not to overcrowd the pan. I could scoop 6 to 8 per tray, with two trays in the oven at a time.
• Cook for 10 or 11 minutes, until the cakes are cooked through but not dry.
• Note: As oven temperatures vary, check the cake whoopie pies after 10 minutes. This recipe was tested by baking two trays at a time, and my top tray baked for10 minutes, whereas the bottom tray needed a total of 11 minutes. These pies will be fully cooked, yet we remove them from the oven before they brown on top to keep the pies pillowy, soft, and moist.
• Once cooked, remove from the oven and then transfer each pie to a cooling rack.
• Continue scooping and baking your batter until the batter is gone- you should have 24 to 28 scoops of whoopie pies per recipe. If you end up with an odd number of cake pies, don't worry! I'm sure someone in your house would love to taste it.
• Allow all the cakes to cool on the cooling rack while you make the filling.

Make the Cream Filling:

• Add 1 cup of shortening (or 6.7 oz, if measuring from a large container) and 1 cup of sugar in a stand mixer fitted with the whip attachment. Mix on medium speed for about 30 seconds to 1 minute until combined. Scrape down the sides using the whisk attachment.
• Note: Animal-based shortening was used and tested in this recipe, providing a better result than plant-based or vegetable-based shortening.
• Pro tip: Detaching the whisk and using it to scrape down the sides is the easiest way to mix this, and it doesn’t dirty another kitchen tool!
• Re-mix the cooked flour/milk mixture to remove any lumps, add this to the shortening /sugar mixture, and add the 2 teaspoons clear vanilla extract.
• Whip on high speed (tested at speed 10) for 8 (up to 10) minutes until it is light and doubled in size. Halfway through the mixing (around minute 4), scrape down the sides and mix the filling a bit using the whisk attachment to ensure all ingredients are incorporated.
• Note: This filling must be whipped on high for a minimum of 6 minutes to start to double in size and incorporate all the ingredients. If it is mixed for less time, the filling could soften and be gritty. Check after 8 minutes to verify the volume, as mixers may vary in speed settings. Adding an extra minute or two can help ensure a whipped doubled filling.
• Add your whipped filling into a piping bag fitted with your favorite cake decorating tip (this recipe was made and photographed using a large back equipped with a star tip) to build the whoopie pies.
• Note: Refrain from refrigerating the filling before building the pies, as the whipped texture will be lost when chilled too long.

Build your Cake Mix Whoopie Pies:

• Take your cooled whoopie pies and line the bottoms on a tray.
• Starting in the center, pipe about 1 to 1.5 tablespoons of filling around the surface of the bottom piece.
• If desired, add 1 teaspoon or more of rainbow sprinkles on top of the filling.
• Take the top piece of the whoopie pie and add a small dot of filling, then place it on top of the filled/sprinkled bottom pie piece, gently pressing the filling out to the edge of the cake.
• Repeat with your remaining cakes, filling on the bottom whoopie pie, adding sprinkles, and topping with another cake piece.
• Serve immediately or chill in the fridge covered for 30 minutes and then serve.
• Note: Some people enjoy a generous amount of filling, yet too much, and you may run out before filling your whoopie pies. Extra filling can be used for creative presentations.
• It is best to fill and create the whoopie pies when the filling has been made. Leftovers can be refrigerated yet this recipe is best enjoyed right after making!