Preheat oven to 350°F, and prepare an 8 X 8-inch baking pan by lining it with parchment. It will allow you to lift the shortbread out of the pan and slice it easily and neatly on a cutting board.
Cream together the butter, powdered sugar and maple sugar until pale and fluffy.
Whisk the flour, cornstarch and salt together, then add to butter mixture. Beat for 1 minute on low in a mixer, then increase the speed to medium-high for 2 minutes.
Press the batter into an even layer in the prepared pan and bake for 20 - 25 minutes until top and edges are lightly golden.
Allow to cool to room temperature in the pan before removing the shortbread in the parchment and slicing into sticks.
Notes
There is a subtle maple syrup flavor in these cookies. If you want a stronger flavor, I suggest using 2 tablespoons of powdered sugar and increasing the maple sugar to 1/4 cup plus 1 tablespoon. Adding any extract to increase the flavor will change the consistency of the shortbread.