Preheat your oven to 375°F.
In a large pot of boiling, salted water, cook your pasta according to package directions.
In the meantime, heat a large saucepan over medium high heat. Add the pancetta, cooking until crisp and the fat has been rendered out. Remove the pancetta using a slotted spoon, leaving the fat behind.
Whisk in the flour, cooking for two minutes, stirring constantly.
Add the milk, about a cup full at a time, letting the mixture thicken and the milk heat as it gets added. Continue whisking frequently. Do not let the mixture come to a boil.
Add both cheese to the milk mixture in batches, about a cup at a time, whisking and letting it melt before adding the next batch.
Add the cayenne, salt and pepper. Taste the sauce for seasoning and adjust as necessary.
In a large mixing bowl (or the large pot you cooked the pasta in) combine the sauce and the pasta. Pour into a large 9x13" pan lined with parchment paper.
Make the breadcrumbs: combine the cubed bread, butter and thyme in a large mixing bowl. Mix with your hands to ensure the butter is evenly distributed. Layer the crumbs over the mac and cheese.
Bake in the oven for 30-45 minutes, until the breadcrumbs are golden brown and the cheese sauce is bubbling. Serve immediately.