Boil salted water and cook the elbow macaroni according to package instructions until al dente. Drain well and set aside.

In a small saucepan over medium heat, melt 1 tablespoon of butter. Whisk in the flour and cook for 1 minute until lightly golden and bubbly.

Gradually whisk in the whole milk, cooking for 2 to 3 minutes until the mixture slightly thickens. Remove from heat and stir in the cheddar, gruyere, garlic powder, paprika, salt, and pepper until perfectly smooth and creamy.

Fold the cooked macaroni into the cheese sauce until well coated, then let it cool for 5 minutes so it thickens and stays together on the bread.

Butter one side of each sourdough slice using the remaining softened butter. Place two slices, butter-side down, on a cutting board and heap the warm mac and cheese evenly onto them.

Top with the remaining bread slices, butter-side up. Heat a large skillet over medium-low heat and cook the sandwiches for 3 to 4 minutes per side, pressing down gently with a spatula, until the bread is golden brown and crispy.

Remove from the skillet and let rest for 1 minute before slicing in half. Serve immediately with a side of tomato soup or a crisp green salad for the ultimate comfort meal.