Chop or crush Oreo cookies. Combine cereal and Oreos in a bowl; set aside.

Butter the bottom and sides or a 11"x 8" pan (or spray with non-stick spray). Set aside.

In a large glass bowl, melt the butter. Add the mini marshmallows. Cook for about 3 minutes, stirring every 30 seconds till mixture is completely smooth.

Stir in the vanilla. Add the cereal/oreo mixture and stir to coat.

Press firmly into the prepared pan. Let cool to room temperature and cut into squares.

Melt the chocolate chips in a small ziplock bag in the microwave at half power. Snip a corner and drizzle the melted chocolate over the bars.