Preheat the oven to 350°F and lightly spray a 13x9-inch baking dish with nonstick cooking spray.
In a large bowl, stir the cake mix, melted butter, and egg together to make the crust. Fold in the strawberries. Press the mixture evenly into the bottom of the pan. It may make it easier to lightly spray your hands with nonstick cooking spray and pat the crust dough down.
In another large bowl, use a mixer to beat the cream cheese until smooth. Add the eggs and mix well. Add the butter and mix well. Gradually add the powdered sugar and mix until combined. Mix in the vanilla. Pour the filling mixture over the crust.
Bake for 40 to 50 minutes or until the edges are just firm and lightly golden brown but the middle is still jiggly. Cool completely before slicing into squares.