Preheat oven to 350 F. In a mixing bowl, combine sugar and graham crumbs. Stir well. Pour the melted butter and stir the mixture to distribute the melted butter. The mixture will be somewhat damp.

Transfer the mixture into the center of an 11-inch tart pan with removable bottom. Spread the mixture all over so that the bottom of the pan is covered. Using the base of a round glass or measuring cup, press the graham mixture against the pan, and work your way up to the sides and press it firmly. The mixture should be pressed tightly so they are tightly packed against the surface and sides of the tart pan. Bake the crust at 350 F for ten minutes, or until the crust is set.

Combine sugar and cornstarch in a medium saucepan. Add the condensed milk and sour cream and stir everything until the mixture is smooth. Set the pan over medium heat. Cook the mixture while stirring constantly until it is thick with almost a spreadable consistency. Turn off heat.

In a small bowl, lightly beat the egg yolks. Add about 1 cup of the hot mixture to the yolks and stir immediately. Return this mixture to the pan. Cook the mixture again over medium heat this time for just about 5 minutes or so, while stirring constantly until it is thick. The mixture should fall back in dollops when you let it fall from a spoon. Turn off heat and allow the mixture to slightly cool.

Divide the custard into two equal portions. To one portion, stir 2 teaspoons of ube extract until the color is uniform. Pour the plain custard over to one side of the tart crust, then pour the ube custard to the other half. Alternately you can add the custards alternately to the crust to create a marbling effect. Use two pieces of clean toothpicks to swirl the mixture around for that swirled effect. Let the tart chill in the fridge for at least two hours before serving.