Line an 8×8-inch square pan with parchment paper, leaving an overhang on two sides to lift the fudge out later. Lightly mist the parchment with nonstick spray if it tends to stick.

In a medium, heavy-bottomed saucepan, add the white chocolate chips, sweetened condensed milk, butter pieces, and fine sea salt.

Set the saucepan over low to medium-low heat. Stir slowly and constantly with a heatproof spatula, scraping the bottom and sides, until the white chocolate is completely melted and the mixture is thick, glossy, and smooth. Do not rush this step; if you see unmelted bits, lower the heat and keep stirring so the white chocolate does not scorch.

Remove the pan from the heat and let it sit for 1–2 minutes to cool slightly. Stir in the vanilla extract, lime zest, and about 1/4 cup of the key lime juice. Stir vigorously until smooth. Taste carefully (it will be warm) and add more lime juice, up to the full 1/3 cup, for a stronger citrus flavor.

If desired, add 1–2 drops of green food coloring and stir well to create a pale green color. Start with a small amount; you can always add another drop if needed.

Quickly pour the warm fudge mixture into the prepared pan. It will begin to firm up as it cools, so work promptly. Use a spatula to spread it into an even layer and smooth the top.

While the top is still soft, sprinkle with extra lime zest, finely crushed graham crackers, or toasted coconut flakes, if using. Gently press the toppings so they adhere.

Refrigerate the pan for at least 2 hours, or until the fudge is fully set and firm to the touch. For very clean cuts, chill 3–4 hours or overnight.

Use the parchment overhang to lift the fudge slab from the pan onto a cutting board. Dip a sharp knife in hot water, wipe it dry, and cut the fudge into 1-inch squares or rectangles, wiping the knife between cuts for neat edges.

Arrange the key lime fudge squares on a serving plate or pack them into tins or boxes. Keep refrigerated until serving so they stay firm and creamy.