Preheat oven to 375.

Line a 9 x 9 inch pan with aluminum foil and grease with butter. Set aside.

In a bowl, combine the sugar and graham crackers crumbs and stir to combine.

Add the melted butter and stir until crumbs are all moistened.

Spread evenly into the baking pan and press until even. Bake at 375 degrees for 4 to 5 minutes or until edge is lightly browned.

Remove from the oven and let cool on a wire rack.

Once crust has cooled, add the white chocolate, sweetened condensed milk, and 1 Tbsp butter to a saucepan.

Cook on low heat just until chocolate is melted and and mixture is smooth. Remove from heat and stir in the zest and juice.

Spread mixture evenly over the crust. Cover and refrigerate for 12 hours.

Remove fudge from the pan using the foil and cut into pieces.

Lift the fudge from pan using edges of foil. Use a damp kitchen towel to clean off the knife after each cut for a clean cut.