Preheat oven to 350°. Line a straight-sided 13" x 9" baking pan with parchment paper, letting the ends create an overhanging edge for easy removal.
In a medium-sized saucepan, combine cranberries, 1/2 cup sugar and 1/4 cup grapefruit juice. Cook over medium-high heat until mixture begins to boil. Turn down heat and simmer until cranberries begin to break down and the sauce becomes thick and syrupy, about 30 minutes. Remove from heat and let cool while you make the shortbread.
In a medium bowl, mix together the butter and 1/4 cup of the sugar. Whisk in the egg and vanilla. Stir in the pancake mix to make a stiff dough. Transfer three-quarters of the mixture to the prepared pan, and press evenly into the bottom. Prick the dough all over with a fork. Refrigerate the pan for 30 minutes until the dough is firm.
Beat cream cheese and sweetened condensed milk in medium bowl; mix until smooth.
Remove crust from refrigerator, bake in preheated oven for 10 minutes.
Pour cream cheese mixture over crust. Spoon cranberry mixture over cream cheese layer. Sprinkle reserved shortbread topping over cranberry mixture.
Bake for 35 to 40 minutes or until center is set and crumb topping is golden. Cool completely in pan on wire rack. Using paper overhang, lift shortbread from pan and transfer to a cutting board. Cut shortbread.