In a pot, heat enough water to cover the pasta plus a few teaspoons salt over high heat until boiling. Add pasta and cook until al dente. Stain and set aside.
In the meantime, in a large pot, melt the butter. Add the garlic and cook for one minute.
Add the cayenne and a big pinch of salt and pepper.
Add the flour and whisk to combine. Cook over medium heat, so it bubbles as it cooks. Stir every so often and allow to cook for a few minutes. This is called a roux. It's the texture of a paste.
Now, pour in the stock and milk. Reduce the heat to a medium/low and whisk so the roux becomes well incorporated.
Stir in the pepper jack cheese and half the cheddar and stir until melted.
Add the pasta and pour into your baking dish. Top with remaining cheese, chives and then breadcrumbs.
Bake (covered in foil) at 350°F for 20 minutes.
Remove the foil and continue to bake for 8-10 minutes until golden brown and edges bubble.
Notes
To make your own breadcrumbs, take 3 pieces of bread and bake them at 350°F until very toasty on both sides. I sometimes do this while the oven preheats and I'm preparing the rest of this recipe. Just keep an eye on it. Once toasted, pulse in a food processor to make breadcrumbs.