Start by preheating your oven to 350°F and preparing a 13x9x2 inch baking pan by lining it with foil and lightly greasing the surface. For the crust, combine 1 1/4 cups of finely crushed graham crackers with 1/4 cup granulated sugar, then mix in 1/3 cup melted butter until the mixture resembles wet sand. Press this mixture evenly onto the bottom of your prepared pan and set aside while you prepare the filling.

To make the filling, beat together 3 (8 ounce) packages of softened cream cheese and 1 1/4 cups packed brown sugar until well mixed. Add 3 eggs one at a time, beating on low speed after each addition just until combined to avoid overmixing. Stir in 3 tablespoons of thawed frozen orange juice concentrate, 1 1/2 teaspoons of finely shredded orange rind, 1/2 teaspoon vanilla, and 1/4 teaspoon salt. Reserve 1 cup of this filling in a separate bowl. In a small saucepan, melt 6 ounces of semisweet chocolate with 2 tablespoons of butter over very low heat, stirring often, then allow to cool slightly before stirring it into the reserved filling.

Carefully spread the chocolate-filled mixture over the crust and bake for 15 minutes. Remove from the oven and pour the remaining plain orange filling over the chocolate layer, spreading it evenly. Bake for an additional 40 to 45 minutes until the center is just set, then remove and cool for 30 minutes. While the bars cool, combine 1 1/2 cups sour cream with 1/3 cup granulated sugar. Spread this sour cream mixture over the cooled bars, then refrigerate for several hours or overnight before cutting into bars.