Heat oven to 350°F (175°C) and butter a 9x13 inch baking dish. Cook macaroni in salted boiling water until just firmer than al dente, about 6-7 minutes. Drain and return to the pot off heat.

In a saucepan over medium heat, melt 4 tbsp butter. Stir in Velveeta until smooth, then whisk in milk gradually. Add cheddar and Monterey Jack in handfuls, stirring until fully melted and glossy.

In a small bowl, whisk eggs with a few spoonfuls of warm cheese sauce. Pour into the pasta along with remaining sauce, salt, and pepper; fold to coat.

Transfer to the prepared dish. Mix breadcrumbs with a little melted butter, then scatter on top.

Bake 30-35 minutes until golden and bubbly. Let rest 5 minutes, then serve hot.