Line a large baking sheet* with parchment paper and spray with nonstick cooking spray. Coarsely chop 12 of the Oreos, then transfer to a large mixing bowl.

Using scissors/kitchen shears coated in nonstick cooking spray, cut 12 peep bunnies into smaller pieces. Add to the bowl of chopped Oreos along with the cereal and stir together; set aside.

In a large saucepan, melt the butter and marshmallows together over low heat until marshmallows are almost melted--it should still be a little lumpy. Remove from the heat and gently fold in the cookie/peeps/cereal mixture until just combined. Try not to spend too much time stirring, which can cause the peeps to bleed and melt too much into the rice krispies--you still want chunks of them. Dump rice krispies out onto prepared pan, and use a silicone spatula or greased hands to press the mixture into an even thin layer.

Melt half of the white chocolate (18 ounces) in the microwave until smooth and pourable. Drizzle white chocolate over the rice krispies, then use a knife or offset spatula to spread the chocolate evenly over everything. Refrigerate until chocolate has set.

Heat the remaining white chocolate in the microwave until smooth. Turn the sheet of rice krispies over onto its chocolate side (which is now set), then drizzle the melted chocolate over the rice krispies, using an offset spatula to spread the chocolate evenly. Immediately top with extra peeps, Oreos, sprinkles, and M&Ms. Refrigerate until chocolate has set, then use a knife to cut bark into several pieces.