Step 1: Prepare the "Pie" Batter: In a large bowl, combine the cake mix, vegetable oil, egg, and milk (or water). Mix until just combined; avoid overmixing. The batter will be thick.
Step 2: Drop onto Baking Sheet: Drop rounded tablespoons of batter onto a parchment-lined baking sheet, leaving about 2 inches between each. These will spread slightly.
Step 3: Bake the "Pies": Bake in a preheated oven at 350F (175C) for 12-15 minutes, or until the edges are lightly golden and a toothpick inserted into the center comes out clean.
Step 4: Cool the "Pies": Remove from the oven and let cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.
Step 5: Prepare the Filling: While the "pies" are cooling, prepare the filling. In a large bowl, beat the softened butter with an electric mixer until light and fluffy.
Step 6: Add Powdered Sugar: Gradually add the powdered sugar, one cup at a time, mixing well after each addition.
Step 7: Add Milk and Vanilla: Add the milk and vanilla extract, and beat until the filling is smooth and creamy. Add more milk, one tablespoon at a time, if needed to reach the desired consistency.
Step 8: Assemble the Whoopie Pies: Once the "pies" are completely cool, turn half of them upside down. Spread a generous amount of filling onto the flat side of each of those "pies". Top with the remaining "pies," flat side down, to create a sandwich.
Step 9: Serve and Enjoy: Serve immediately or store in an airtight container in the refrigerator.
Tips and Substitutions: For a richer flavor, use melted butter instead of oil in the "pie" batter. Cake mix flavors can be varied to taste (chocolate, vanilla, red velvet, etc.). Filling variations include using cream cheese frosting or adding peanut butter to the buttercream. For a gluten-free version, use a gluten-free cake mix.