Position a rack in the middle of the oven. Heat oven to 300° F. Line two baking sheets with parchment paper.
In a medium bowl, sift both flours, cornstarch, baking powder and salt and set aside. In a large bowl and using an electric mixer on medium speed, beat the Kerrygold Pure Irish Butter, powdered sugar and vanilla until smooth and lightened slightly in color, 1 minute. Stop the mixer and scrape the sides of the bowl as needed during the mixing. Mix in the flour mixture just until the flour is incorporated and a smooth dough forms.
For each cookie, roll a level tablespoon of dough between the palms of your hands into a smooth ball. Place the cookies on the prepared baking sheets, spacing them 2 inches apart. Use a fork to gently flatten the cookies to 1 1/4-inch disks and to make an impression of the fork tines in the top of each.
Bake the cookies one sheet at a time until the tops feel firm but do not darken, about 30 minutes. The cookie bottoms will be lightly browned. Cool the cookies on the baking sheets for 10 minutes. Use a wide metal spatula to transfer the cookies to a wire rack to cool completely.
In a medium bowl, use a large spoon to stir the Kerrygold Pure Irish Butter, powdered sugar and vanilla until smooth. Stir in melted chocolate, if desired. Turn half of the cooled cookies bottom side up. Use a thin metal spatula to spread a rounded teaspoon of filling evenly over the flat bottom of each. Gently press the flat bottom of the remaining cookies on the filling. Serve at room temperature. Cookies can be stored in a tightly covered container at room temperature for up to 5 days.