FILLING

Whisk 5 tbsp flour and 1 cup milk over low heat until it forms a thick paste. This will only take a couple of minutes, so vigorously whisk together, ensuring all clumps are gone, and remove the mixture from heat as soon as it thickens. Don't cook too long, or the paste will start to burn and get clumpy.

Remove paste from pan into a small bowl and allow to cool down fully

In the bowl of a stand mixer, mix shortening or butter, powdered sugar, salt, and vanilla until well-blended

Fold in the paste mixture and beat on high for 4-5 minutes or until light and fluffy, ensuring all the paste isn't stuck to the bottom of the bowl. The longer you mix, the lighter and fluffier your filling will be.

Add a dollop of filling to the flat side of one cake and place a second cake on top (flat side down) to form a sandwich. Press down on one of the whoopie cake halves. Don't press too hard, or the filling will squeeze out. Repeat with the rest.

Place the M&M's on top of the filling around the outside of each pie. You can also drizzle extra chocolate or confectioner’s sugar on the tops of whoopie pies.