Line a cookie sheet or serving plate with wax paper -OR-Spray 8" square pan with cooking spray. Set aside. (Use the pan if you want to serve as bars instead of "lumps")
Crush or process cookies into a fine powder.
Over medium/low heat, melt butter and marshmallows in a saucepan until smooth.
Remove from heat and stir in cookie crumbs. Mix well.
Use a spoon (it's sticky, so a cookie scoop won't work) and drop into piles onto wax paper. Shape into lumps if necessary. -OR-Press into prepared pan and allow to cool.