Preheat the oven to 350°F. Spray a 2-quart baking dish with non-stick cooking spray.
Cook the macaroni according to package directions for al dente consistency. Drain, rinse with cold water to stop the cooking process, and set aside.
In a large saucepan over medium heat, melt 3 tablespoons of butter. Add the onion and saute for 2 minutes, until softened.Add the flour, powdered mustard, and paprika, whisking until the mixture is smooth.
Slowly pour in the heavy cream, whisking until fully incorporated. Add the bay leaf then reduce heat to low. Simmer for 5 minutes, until sauce thickens, then remove the bay leaf and turn off the heat.
In a small bowl, whisk together the liquid egg substitute and about 1 tablespoon of the hot creamer mixture. (This step, known as 'tempering' raises the temperature of the egg and prevents it from scrambling when it hits the hot sauce.) Gently stir into the milk mixture in the saucepan.
Add the cheese, stirring frequently until melted. Season with salt and pepper.
Fold the cooked macaroni into the cheese sauce, stirring to fully coat. Pour into the prepared baking dish.
In a small skillet over medium heat, melt the remaining 3 tablespoons of butter. Add the panko breadcrumbs and toss to coat. Top the macaroni with an even layer of breadcrumbs.
Bake, uncovered, for 30 minutes until golden and bubbly. Allow to rest 10 minutes then serve hot.