Line a 9x13" pan with tin foil and spray with non stick spray. Set aside.
In a large microwave-safe bowl, combine butter and marshmallows. Microwave on high for 1 minute, stir. Microwave on high for another minute if necessary to melt the marshmallows. Stir until smooth.
Add in cocoa powder and ½ cup chocolate chips and stir until smooth and chocolate is melted (work quickly). Stir in Krispie rice cereal and salt until totally combined. Pour into prepared pan and smooth with wet hands and set aside.
Truffle layer:In a small pot, heat cream until bubbles appear around the edges. Remove from heat and stir in 3½ cups chocolate until smooth. Let sit for 5-10 minutes to thicken and cool slightly before pouring over the Krispie treat layer. Sprinkle with desired toppings and refrigerate until set.