Preheat the oven to 325°F. Line a 8×8-inch baking dish with parchment paper and/or spray with non-stick cooking spray.
For the Lemon Blueberry Muffin Layer: Using a handheld or stand mixer, beat the butter, sweetener, brown sugar, egg beaters, vanilla, lemon zest, lemon juice, and milk together until combined. Scrape down the sides and up the bottom of the bowl as needed. Beat in the flour, baking powder, and salt on low speed, then fold in the blueberries. Add the lemon blueberry layer to the bottom of the prepared dish and sprinkle with flake sugar, if desired.
For the Blondies: In a large bowl, add the protein powder, flour, sweetener, brown sugar, baking powder, baking soda, and salt. Stir dry ingredients, until they are mixed well. Then add the yogurt, applesauce, vanilla, butter, and melted peanut butter chips. Using a hand or stand mixer, mix to combine. Add the blondie batter overtop the lemon blueberry layer and spread out evenly.
Bake: Bake for 40-45 minutes, or until a toothpick inserted in the center comes out mostly clean. It will still cook some in the hot dish when you remove it from the over.
Cool: Let blondies cool in the pan for 15 minutes. If you wish to eat warm, cut into bars. If you prefer them cold, go ahead and store them in the fridge, covered, and cut into bars when you are ready to eat. They will be easier to cut the longer they have set.
Optional Lemon Glaze: If you wish to add the glaze, simply whisk the confectioners' sugar, lemon juice, and milk together until smooth. Drizzle over blondies once they have cooled to room temperature or after refrigerating.