In a large bowl, mix the cake mix, butter, milk, salt and eggs until the consistency of a batter.
In a second bowl, mix the sprinkles, chocolate candies and vanilla morsels. Fold two-thirds of the candy mix into the batter.
Coat a 9-by-13-inch pan with baking spray. Add the batter to the pan and smooth the top. Sprinkle over the remaining candy mix and bake until golden and set in the middle, 25 to 27 minutes. Allow to cool completely, about 1 hour.
Cut into 16 bars. Store extras in an airtight container at room temperature.