Melt the butter in a large pot over low heat. Add the marshmallows, and stir until they're completely melted. Remove from heat, and stir in the Rice Krispies cereal.
Let the cereal sit for just a few minutes to let it cool down a little. While they're cooling, place the Cadbury eggs in a plastic bag, then use a rolling pin to smash them into large pieces. Slowly fold in the chocolate pieces into the Rice Krispie treats mixture.
Pour the mixture into the buttered pan. Butter your hands, then press down the mixture until the top is smooth. Or, use a piece of wax paper to press it down. Cut into pieces.