Place the sweetened condensed milk, chocolate chips, margarine and vanilla is a large microwave safe bowl. Microwave on high one minute, then stir, then microwave one minute again. Continue until the ingredients are melted and mix until smooth.
Once the chocolate mixture is melted and smooth then stir in the nuts and marshmallows.
Stir to combine well and then spread into a 9×13 inch greased pan.
Let cool/chill at least 2 hours and then cut into squares once firm.