Melt the butter in a large saucepan over low heat. Add the mini marshmallows and stir until fully melted and smooth. Remove the pan from heat and quickly fold in the Rice Krispies cereal until evenly coated.

Press the mixture firmly into a parchment lined 8 by 8 inch pan using a buttered spatula or your hands. Let it cool and set for 20 minutes, then cut into four squares.

Place the dark chocolate chips in a microwave safe bowl and heat in 20 second bursts, stirring between each, until melted. Do the same with the white chocolate chips in a separate bowl. Pipe or drizzle the dark chocolate over each square in a loose spiral. Add short lines of white chocolate from the center outward to form the web pattern. Let the chocolate harden before serving.