Spray 8" x 12" baking dish with canola oil or non-stick cooking spray. Set aside.

Melt butter in large saucepan over low heat. Add marshmallows and stir until melted.

At the same time, place milk chocolate and white chocolate chips in a small saucepan and melt over low heat. You could also melt them in the microwave, but I find that the white chocolate is difficult to melt that way. 

Use half of the Rice Krispies mixture to create an even layer in the prepared pan. Spread melted chocolate on top. With the remaining Rice Krispies mixture, create an even layer on top. 

Place in refrigerator for about 2 hours until chocolate is solid. Cut and serve. Best stored at room temperature in air-tight container.