I found this recipe from dozenflours.com. I thought the fudge turned out really well, it's a little sweet, but isn't fudge supposed to be? Here's the recipe:
Graham Cracker Crust:
1/3 cup butter
1/4 cup sugar
1-1/4 cups finely crushed graham crackers
3 cups white chocolate chips
14 oz can sweetened condensed milk
1 tbsp lime zest - remember to clean those limes!
3 tbsp lime juice
Line a square 8x8 baking pan with foil/parchment paper (extend foil over edges for easy removal of fudge later). Butter sides and bottoms of foil, set aside.
Preheat oven to 375 degrees. Melt butter in the microwave and then pour it into a medium sized bowl. Add sugar and graham crackers and mix well, no lumps! Pour mixture into lined pan and press down with fingers so as to make a sturdy crust. Cook in oven for about 5 or 6 minutes, or until lightly browned. Remove from oven and let cool.
In a large pan (preferrably non-stick), add sweetened condensed milk and white chocolate. Stir almost constantly over low heat until chocolate is melted and there are no lumps present. Remove from heat and add lime zest and juice. Stir well. Pour mixture onto graham cracker crust.
Cover with foil and chill, preferrably overnight. Lift foil to remove fudge from pan, and peel off of the sides of the fudge. Cut the fudge into squares; you may need to press a little harder to cut through the crust. To keep each cut clean, wipe your knife off with a paper towel soaked with warm water between cuts.
Store in an airtight container at room temperature for up to one week.