In a large bowl, mix the box cake, eggs and oil to create a gooey batter. Roll into balls and place on a cookie sheet a couple of inches apart. Bake according to box cake package directions. Remove from oven and let cool.
To make Strawberry Filling:
In another large bowl, beat butter until creamy. Add powdered sugar and vanilla and beat until fluffy. Add the diced strawberries and fold with a spatula until well incorporated.
To assemble Whoopie Pies:
Take one cookie and top with filling. Add strawberry slices and top with another cookie to create the whoopie sandwich.
*We used Duncan Hines Classic Yellow Cake Mix. Nutrition information calculated based on a 12 pie yield.