Step 1: Preheat your oven to 350°F (175°C). Line an 8x8-inch (or similar small square) baking dish with parchment paper, leaving an overhang on two sides for easy removal, and lightly coat with cooking spray.
Step 2: In a medium mixing bowl, combine the almond flour, coconut flour, 2 tbsp granulated erythritol, and salt. Mix well. Pour in the melted coconut oil and mix with a fork until the mixture resembles coarse, moist crumbs.
Step 3: Press the crust mixture evenly and firmly into the bottom of the prepared baking dish to form a compact layer.
Step 4: Bake for 10-12 minutes, or until the crust is lightly golden brown. Remove from the oven and let it cool slightly on a wire rack while you prepare the filling.
Step 5: In a large mixing bowl, whisk together the egg, ¼ cup granulated erythritol, almond milk, heavy cream, and ½ tsp vanilla extract until smooth and well combined.
Step 6: Lightly sprinkle the xanthan gum over the egg mixture. Whisk vigorously for about 30 seconds to fully incorporate it and prevent lumps.
Step 7: Gently fold in the diced fresh or frozen (unthawed) rhubarb into the custard mixture.
Step 8: Carefully pour the rhubarb custard mixture evenly over the partially baked crust in the baking dish.
Step 9: Return the dish to the oven and bake for 30-40 minutes, or until the custard has set. A knife inserted into the center should come out clean. The top may be lightly golden.
Step 10: Remove the bars from the oven and let them cool completely on a wire rack at room temperature. This is crucial for the custard to fully set.
Step 11: While the bars are cooling, prepare the topping. In a medium mixing bowl, beat the softened cream cheese with the powdered erythritol and ¼ tsp vanilla extract until smooth and creamy.
Step 12: Gently fold in the thawed sugar-free Cool Whip until it's fully combined and the topping is light and fluffy.
Step 13: Once the rhubarb custard layer is completely cooled (and firm to the touch), spread the cream cheese topping evenly over it.
Step 14: Cover the baking dish and refrigerate the Keto Rhubarb Dream Bars for at least 2-3 hours (preferably longer, or overnight) to allow them to set completely before slicing and serving. Use the parchment paper overhang to lift the bars out before cutting.