Step 1

Line a 9x9-in. baking pan with aluminum foil and spray with cooking spray. Set aside.

Step 2

If making a crust (otherwise skip to step 3), preheat oven to 375°F. Place the graham crackers in a food processor and pulse until they resemble fine crumbs. Add the butter and sugar and pulse just until mixed. Press the mixture evenly into the bottom of the prepared pan. Bake for 5-7 minutes or until the edges are lightly browned. Remove from the oven and allow to cool while you prepare the fudge layer.

Step 3

In a large microwave-safe bowl, combine the white chips and sweetened condensed milk. Microwave in 30-second increments, stirring after each cooking time until the chips are melted and the mixture is smooth. Add the zest and juice. Working quickly, mix until thoroughly combined. Evenly pour the fudge into the prepared pan over the graham cracker crust, if using, and smooth it out. Refrigerate for 2-3 hours or until the fudge is completely set.

Step 4

Remove the fudge from the pan using the foil to lift it out. Use a sharp knife and cut the fudge into 1-inch squares.

Step 5

Store fudge in an airtight container in the refrigerator. Use within 2 weeks.