Bring a medium pot of salted water to a boil and cook the elbow macaroni according to package directions until al dente, then drain and set aside.

While the pasta cooks, fry the bacon in a large skillet over medium heat until crispy, about 8-10 minutes, then drain on paper towels and crumble into small pieces.

In a medium saucepan, melt 2 tablespoons of butter over medium heat, whisk in the flour, and cook for 1 minute to create a roux.

Gradually whisk in the milk and simmer until the sauce thickens slightly, about 3-4 minutes.

Remove the sauce from heat and stir in the shredded cheddar and mozzarella cheeses until melted and smooth.

Fold the cooked macaroni and crumbled bacon into the cheese sauce until evenly coated, then season with salt and pepper to taste.

Allow the mac and cheese mixture to cool for about 5-10 minutes so it firms up slightly, making it easier to handle.

Spread the softened butter on one side of each slice of sourdough bread.

Place 4 slices of bread, butter-side down, on a clean surface and pile the mac and cheese mixture generously onto each slice.

Top with the remaining bread slices, butter-side up, pressing down gently to secure the filling.

Heat a large skillet or griddle over medium-low heat.

Cook the sandwiches for 3-4 minutes per side, pressing down occasionally with a spatula, until the bread is golden brown and crispy.

Remove from the pan and let the sandwiches rest for 1 minute before slicing in half to serve warm.