Using a stand mixer fitted with the paddle attachment, cream together the butter, 1/2 cup of granulated sugar, and confectioners' sugar on medium speed for about 5 minutes, or until the mixture is light and fluffy.
Beat in the egg yolks and vanilla on medium speed for 2-3 minutes, or until thoroughly combined.
In a medium bowl, sift or whisk together the flours, baking powder, and salt. On low speed, gradually add the flour mixture to the butter mixture until the flour mixture is totally incorporated and the dough is evenly mixed.
Scrape the dough onto two sheets of plastic wrap, dividing evenly. Refrigerate for 30 minutes or until the dough if firm.
Preheat the oven to 350°F.
Press one of the shortbread disks into a greased 9x13 inch baking pan. Bake for 20 minutes or until the shortbread is light brown.
Meanwhile, in a bowl combine the blueberries, the remaining 1/2 cup of granulated sugar, cornstarch, lemon juice and Sumac. Toss to coat.
Remove the shortbread from the oven (leaving the oven set at 350°F), let cool for 10-15 minutes. Then top the still warm shortbread with the blueberry mixture. Spread it in an even layer.
Remove the remaining shortbread disk from the refrigerator and using a box grater or your hands; evenly spread the shortbread dough over the blueberries.
Bake for 20-25 minutes or until the top is lightly browned. Cool completely. Serve with additional sifted confectioners’ sugar.