Preheat oven to 350°F (175°C). Line a 8x8-inch pan with parchment paper; set aside.
In a medium-sized bowl, combine together the butter, apple sauce, sugar (or sweetener), flour and cornstarch. Using a fork or your hands, blend ingredients together until sandy in consistency.
Pour into lined pan and press down evenly to form a base; bake for 20 minutes, or until firm and slightly golden.
While the base is baking, whisk the filling ingredients until well combined. As soon as the base comes out of the oven, give the filling one more whisk and pour the filling over the base.
Return to the oven and bake for an additional 20-25 minutes, or until just set.
Allow to cool for an hour before refrigerating for about 2 hours. Dust with confectioners, powdered or icing sugar (or powdered sweetener).
Cut into 20 squares and serve.