Layer a key lime pie-like topping over a classic shortbread recipe to create these pecan shortbread bars that are sure to fly off your dessert spread.
Preheat oven to 350°F. Line a 13x9x2-inch baking pan with foil, extending the foil over the edges of the pan. Grease foil; set pan aside.
For crust, in a large bowl stir together flour, powdered sugar, cornstarch, and the 1/4 teaspoon salt. Using a pastry blender, cut in the 3/4 cup butter until mixture resembles coarse crumbs. Stir in pecans. Press mixture evenly into bottom of the prepared pan. Bake for 18 to 20 minutes or until edges are golden brown. Remove from oven. Reduce oven temperature to 325°F.
Meanwhile, for filling, in a medium saucepan combine eggs, egg yolks, granulated sugar, lime peel, lime juice, and dash salt. Stir in the 1/2 cup butter. Cook and stir over medium-low heat about 10 minutes or until mixture thickens. Pour filling over hot crust, spreading evenly. (If desired, strain filling over crust through a fine-mesh sieve.)
Bake about 10 minutes or just until filling is set. Cool completely in pan on a wire rack. Use foil to lift uncut bars out of pan. Cut into bars. If desired, spoon Sweetened Mascarpone Cheese onto bars and top with lime slices and/or wedges.
Place bars in a single layer in an airtight container; cover. Store in the refrigerator for up to 2 days.
In a small bowl combine mascarpone cheese and powdered sugar. Whisk together until smooth.