Line a 9 x 13 rectangular pan with parchment paper or butter the sides and top to prevent sticking. If you prefer thicker treats, opt for a 9-inch square pan instead.
In a large pot or saucepan over medium heat, melt the butter.
Once melted, add the vanilla extract and mix well.
Add all the marshmallows and continue stirring until they melt completely into the butter, leaving no clumps.
Pour in the cereal and chopped Oreos, stirring until everything is evenly mixed.
Transfer the mixture to the pan and use a rubber spatula or your fingers to gently press it down evenly. Be careful not to pack it too tightly; we want them fluffy! If it sticks too much, dampen the spatula or your fingers with a little water.
Let the treats cool for 30 minutes to an hour until they set, then slice into squares.
Melt the chocolate according to package instructions, then dip the bottom of each square into the chocolate and set them on a parchment-lined tray. Drizzle the tops with a thin layer of white chocolate.
Once the chocolate hardens, dig in and enjoy!