Preheat oven to 350°F. Line an 8x8-inch baking dish with parchment paper.

Roughly chop the pecans. Spread the chopped pecans on a baking sheet. Bake them for 5-6 minutes, or until lightly toasted. Remove from the oven and let them cool completely (about 30 minutes).

In the bowl of a stand mixer with a paddle attachment, mix the flour, cornstarch, sugar, and salt at low speed until combined (about 5 seconds).

Cut the cold butter into ½-inch cubes. Place the butter cubes in a small bowl and toss with ¼ cup of the flour mixture.

Add the floured butter (with any extra flour) to the bowl of the stand mixer. Mix on low speed until the dough is light yellow in color and small crumbs (about 4 minutes).

Remove bowl from stand mixer. Use your hands to toss the mixture and rub any remaining butter bits into the flour mixture.

Press the shortbread mixture firmly and evenly into the lined pan, smoothing it with the back of a spoon.

Bake 25 minutes, or until very light brown on top.

In a large bowl of a stand mixer with a paddle attachment, add eggs, corn syrup, granulated sugar, brown sugar, butter, and vanilla. Mix well on medium speed (about 2 minutes).

Fold in the toasted pecans. Spread the mixture evenly over the top of the shortbread crust, making sure to get all the way to the edges.

Bake 35-40 minutes, or until the edges are set and the top is dark brown. (The center will jiggle slightly.)

Remove the bars from the oven and let them cool completely before cutting and serving. The bars can also be refrigerated overnight before serving.