Grease a 9×13” baking pan lightly with butter and set aside.

Place your Oreo cookies in a large Ziploc bag and use a rolling pin to break into pieces (don’t pulverize them completely to crumbs, you want discernible pieces!). If desired, transfer to a fine mesh strainer to sift out any small crumbs (this will keep your rice krispie treats from turning gray from the tiny crumbs and is purely optional and done for aesthetic reasons). Set aside.

Melt butter in large pot over medium-low heat. Add 10 oz (5 cups/285g) of your mini marshmallows and cook, stirring frequently, until just melted.

Stir in vanilla extract then add cereal and stir until combined.

Add remaining 5 oz (2 ½ cups/140g) mini marshmallows and broken Oreos and stir briefly until just-combined.

Immediately pour into prepared baking pan and smooth the top with a spatula (do not firmly press the cereal into the pan or it will make the treats tough). Allow to cool completely before cutting and serving.