Combine 12 oz. white chocolate and 1/2 can sweetened condensed milk in a microwave-safe bowl.
Microwave 2-3 minutes or until completely melted, stirring after each minute.
Pour mixture into 9x13 inch pan lined with aluminum foil and refrigerate until set.
In a medium mixing bowl, combine butter, sugar, milk, and vanilla and stir until smooth and creamy.
Add flour and stir until completely incorporated.
Stir in 1 c. m&m's.
Press cookie dough evenly over fudge (once fudge is set).
Prepare top layer of fudge in the same way you prepared the bottom layer.
Pour over cookie dough layer; top with remaining m&m's.
Refrigerate until set.