Preheat the Oven: Preheat the oven to 350°F. Line two muffin tins with cupcake liners and set them aside.
Combine the Wet Ingredients: In a large mixing bowl, add the milk, eggs, and melted butter. Mix well until the ingredients are combined.
Prepare the Cupcake Batter: Add the white or yellow cake mix and instant butterscotch pudding mix to the bowl. Whisk for 2 minutes, or until the batter is thoroughly combined and smooth.
Fill the Cupcake Liners: Divide the batter evenly among the prepared cupcake liners, filling each one approximately two-thirds full.
Bake the Cupcakes: Place the muffin tins in the preheated oven and bake for 17–20 minutes. Check doneness by inserting a toothpick into the center of a cupcake; it should come out clean.
Cool Completely: Remove the cupcakes from the oven and transfer them to a wire cooling rack. Allow them to cool completely before adding the frosting.
Beat the Butter: To prepare the frosting, place the softened butter in a large bowl. Using an electric mixer on medium speed, beat until the butter is smooth.
Add the Vanilla and Sugar: Add the vanilla extract and powdered sugar to the butter. Beat until the frosting mixture is smooth and well combined.
Adjust the Frosting Consistency: Add 1–2 tablespoons of milk and beat on medium-high speed until the frosting reaches your desired consistency.
Frost the Cupcakes: Transfer the vanilla buttercream to a piping bag and pipe it onto the completely cooled cupcakes.
Serve and Enjoy: Serve the frosted butterscotch cupcakes and enjoy their soft texture, sweet butterscotch flavor, and creamy vanilla buttercream topping.