Line an 8x8-inch metal baking dish with foil; set aside.
In a large, microwavable bowl, combine white chocolate chips and sweetened condensed milk. Mix to combine.
Microwave in 30 second increments, stirring well in between, until the chocolate is just melted.
Note: white chocolate won't melt like regular milk or semi sweet chocolate, it will be more paste-like and very thick. Heat the chocolate until you cannot see any defined chips and mixture is mostly homogeneous. Be careful not to overcook!
Add lemon or lime zest and juice and mix until smooth. You should now have a spreadable consistency.
Pour the fudge into the lined 8x8-inch pan and spread with a rubber spatula.
Let cool until completely set, then remove from the pan and cut into squares.