In medium bowl, sift together cornstarch, flour, baking powder and salt; set aside.
In medium mixing bowl, using electric mixer or whisk, mix together butter and sugar until thoroughly combined, smooth and fluffy, about 30 seconds. Add egg yolks and bitter orange; continue mixing until well incorporated. Add reserved flour mixture, mixing and scraping edges of bowl with spatula, until mixture is smooth, and dough comes together, pulling away from bottom and sides of bowl. Using hands, shape dough into disc. Wrap disc in plastic wrap. Refrigerate until cold and firm, at least 30 minutes, or up to 24 hours.
Heat oven to 350°F. On lightly-floured work surface, evenly roll dough to ½” thickness. Using 1½” round cookie cutters, cut dough into circles. Gather scraps of dough; reroll to ½” thickness and cut into circles; transfer circles to parchment-lined baking sheets; refrigerate until dough is cold and firm, about 10 minutes. Discard any remaining dough. Bake cookies until just set, 10-12 minutes (there should be no brown spots). Transfer cookies to cooling racks; let sit until cool.
To assemble cookies, flip one cookie upside down. Using small spoon, spread chilled dulce de leche onto center of cookie; top with second cookie, pressing gently so dulce de leche peaks out sides. Roll edges of cookie in shredded coconut, if desired. Transfer to plate; repeat with remaining cookies, dulce de leche and coconut. Transfer cookies to refrigerator until ducle de leche is set, about 15 minutes.
Making your own dulce de leche is as easy as pie when you use GOYA® Sweetened and Condensed Milk! To do, heat oven to 350°F. Pour 1 can (14 oz.) GOYA® Sweetened and Condensed Milk into 9” pie dish. Carefully set pie dish inside large, deep baking dish filled half way with hot water (this hot water bath will help the condensed milk to gently and evenly caramelize in the oven). Tightly cover baking dish with foil. Cook until milk turns deep caramel-brown, about 2 hours. Remove pie dish from water bath; set aside until cool. Transfer dulce de leche to pastry bag fitted with small round tip, or to sturdy zip-top bag. Transfer to refrigerator until cold and firm. (Note, dulce de leche can be made up to 1 week in advance, when stored, covered, in refrigerator).