Melt chocolate, butter and corn syrup in a saucepan over low heat, stirring often.
Reserve ½ cup of this mixture for topping the brownies later.
Put all the cookies in a Ziploc bag and break them into pieces with a rolling pin.
Add the cookies and marshmallows to the chocolate mixture and stir.
Prepare a 9-inch baking pan by lining it with parchment paper.
Spread mixture in the bottom of pan and press down evenly with your hands. Top with remaining chocolate mixture and spread evenly over the top.
Place in refrigerator for at least 30 minutes.
When set, remove from pan by pulling parchment paper out of the pan, removing all the brownies.
Sprinkle with powdered sugar that has been pushed through a fine mesh strainer.
Cut into squares or triangles.