Cook the penne pasta in a large saucepan of boiling salted water until pasta is cooked (cook according to the instructions on the pack), then drain water and set aside.
In a saucepan, melt the 100g butter over a medium heat then mix in the flour and cook for about 3 minutes to form a roux (thick paste of flour and butter). Add the cayenne, black pepper and salt.
Preheat the oven to 180 degrees C.
Gradually pour the milk into the roux, and stir (or whisk) continuously for about 5 to 10 minutes until sauce is thickened. Remove the saucepan from the stove and add half of the cheese (both red Leicester and cheddar) and stir until it is combined with your sauce. Add the teaspoon of Dijon mustard.
Mix the penne pasta with your cheese sauce and transfer all into an oven proof dish. Sprinkle remaining cheese on top. Mix breadcrumbs with the 1 tablespoon of melted butter and sprinkle on top of the pasta.
Bake in the oven until breadcrumbs are browned slightly, about 15 to 20 minutes. Remove from the oven and serve hot.