Preheat your oven to 300°F (150°C) and line a baking sheet with parchment paper.
In a mixing bowl, combine the egg yolks, softened cream cheese, baking powder, and sweetener (if using). Mix until smooth and well combined.
In another clean bowl, beat the egg whites with a pinch of salt using a hand mixer or stand mixer until stiff peaks form.
Gently fold the beaten egg whites into the yolk mixture, being careful not to deflate the egg whites completely.
Divide the cloud bread batter into three equal portions and spoon them onto the prepared baking sheet, forming three round cloud bread circles, each about 4 inches in diameter.
Bake the cloud bread in the preheated oven for 20-25 minutes or until they turn golden brown. Remove from the oven and let them cool on a wire rack.
While the cloud bread is cooling, cook the bacon in a frying pan until crispy. Set it aside on a paper towel-lined plate.
In the same pan, melt the unsalted butter over medium heat. Crack the egg into the pan and cook it to your desired doneness (fried or scrambled). Season with salt and pepper to taste.
To assemble the sandwich, place one cloud bread circle on a plate. Top it with the cooked egg, cheddar cheese slice, bacon, and fresh spinach or lettuce (if using).
Spread mayonnaise on the second cloud bread circle and place it on top of the filling to create a sandwich.
Enjoy your delicious Keto Breakfast Sandwich with Cloud Bread!