Melt butter and marshmallows in the microwave for 2 minutes, stir, then microwave again for 1 minute. Stir until marshmallows are completely melted.
Add Rice Krispies and stir a few times. Add the candy and fold into cereal and marshmallow mixture.
Press into a foil lined 9" x 12" pan. For thicker treats, use an 11" x 7" pan. Press any remaining candies into the top if desired.
In a small, microwave safe bowl, melt the remaining Hershey bars plus the oil for 30 seconds, stirring in ten second intervals, stirring after each time. Stop when the candy is almost melted and stir until smooth. Drizzle over treats.
Place in the fridge to set melted candy. Peel off the foil and cut into squares.