In a medium sized bowl, whisk together the flour, cornstarch and salt. Set aside.
In the bowl of your electric mixer (or with a hand mixer), beat the powdered erythritol, butter, orange extract, and wild orange essentail oil (if using) until creamy and smooth, about 2 minutes. Add the flour mixture and beat just until incorporated. Cover and refrigerate the dough until firm (at least an hour).
Heat oven to 350 degrees F. and place rack in center of oven. Line two baking sheets with silicone baking mats or parchment paper.
Form cold dough into 1 inch balls and place them on the prepared baking sheets, spacing about 1 inch apart. Bake for 10 - 12 minutes, or until the edges of the cookies just start to brown.
Remove pans from oven and place on a wire rack to cool for about 3 minutes. Then transfer the cookies to a wire cooking rack that is placed over a sheet of parchment paper to cool completely.
Once the cookies are completely cool, put the powdered erythritol into a bowl and roll the cookies through the it a few at a time, placing them back onto the wire cooling rack after they've been coated. Place the remaining powdered erythritol from the bowl into a fine strainer or sieve and sprinkle the tops of the cookies with another coating.
If desired, just before serving, sprinkle the tops of the cookies with more powdered erythritol.
Notes
These cookies store very well.Place in an airtight container between sheets of waxed paper and they will keep well for a couple of weeks. Freezing them is not recommended unless you do so before coating the cookies with any powdered erythritol.