Mix cake mix with melted butter and condensed milk. Spread out 1/2 the mixture into a 9x13 pan. Bake at 350 degrees for 10 minutes. Slice candy bars into little slices and lay these on top of the baked crust. With the remaining batter, crumble it on top of the candy bars. It will have a crumbly texture. No need to spread it on top to make it smooth. Put it back into the oven and bake it for another 20 minutes.