Line a 9x9-inch baking pan with parchment paper and spray with cooking spray. Set aside.
In a large microwave-safe bowl, mix the white chips and sweetened condensed milk together, and microwave on high for 30 seconds. Stir. Continue microwaving in 30 second increments, stirring after each time, until the chocolate is melted and the mixture is smooth. Add the zest of one lemon, the lemon juice, lemon extract, and salt. Mix until thoroughly combined.
Pour the fudge into the prepared pan, and smooth the top. Sprinkle the remaining lemon zest over the top. Refrigerate 2-3 hours or until the fudge is completely set.
Remove the fudge from the pan using the parchment paper to lift it out. Use a sharp knife and cut the fudge into squares.
Store fudge in an airtight container in the refrigerator.
Cut the fudge into whatever size squares you'd like. If you do 3 cuts in each direction it's really easy to cut it (in half and then in half again each direction), but this yields 16 large pieces of fudge. If you make 4 cuts in each direction, it's a little more difficult but makes 25 nice sized pieces.