Heat the oven to 375°F and grease a 24-cup mini-muffin pan or a 12-cup standard muffin pan with cooking spray or butter.

Cook the elbow macaroni in salted water until just under al dente, usually about 1 minute less than the package directions. Drain it very well so excess water does not thin the cheese sauce.

Cook the bacon until crisp, drain it, and crumble it into small pieces. Reserve 1 tablespoon of bacon fat if desired for extra smoky flavor.

Melt the unsalted butter in a saucepan over medium heat. Whisk in the flour and cook for about 1 minute to make a smooth roux.

Gradually whisk in the whole milk. Cook for 3–5 minutes, stirring often, until the sauce is slightly thickened.

Remove the saucepan from the heat and stir in the cheddar, Monterey Jack or mozzarella, Parmesan, kosher salt, and black pepper until smooth.

Combine the cheese sauce with the drained macaroni and let the mixture cool for about 5 minutes. Stir in the lightly beaten egg and most of the crumbled bacon only after the mixture has cooled slightly.

Spoon the macaroni mixture firmly into the prepared muffin cups, dividing it evenly. Pressing the mixture down helps the bites hold together after baking.

Sprinkle the reserved bacon over the tops and add the plain panko breadcrumbs if using. The panko creates a light, crisp contrast against the creamy pasta.

Bake for 18–22 minutes, or until the tops are golden and the bites feel set. Cool them in the pan for 5–10 minutes, then run a thin knife around the edges if necessary before removing.