Step 1: Line an 8x8 inch baking pan with parchment paper, allowing some overhang on the sides to easily lift the fudge later. Set aside.
Step 2: In a large microwave-safe bowl, combine the sweetened condensed milk, white chocolate chips, and unsalted butter. Microwave on high in 30-second intervals, stirring well after each, until the mixture is completely melted and smooth. This usually takes 1.5 to 2 minutes total.
Step 3: Remove the bowl from the microwave. Stir in the fresh lemon juice and lemon zest until well combined. If desired, add 2-3 drops of yellow food coloring and mix until the color is uniform.
Step 4: Pour the lemon fudge mixture into the prepared 8x8 inch pan. Spread evenly with a spatula.
Step 5: Refrigerate for at least 2 hours, or until the fudge is firm enough to slice.
Step 6: Once firm, use the parchment paper overhang to lift the fudge from the pan. Place it on a cutting board and cut into 36 small squares. Serve immediately or store as directed.