I'm just going to say it: this is the best macaroni and cheese I've ever eaten—better than the macaroni and cheese I ate as a child; better than the macaroni and cheese I brought my own children up on when they were little (they don't agree); better than any fancy restaurant macaroni and cheese with white truffle or lobster; better than any macaroni and cheese I have loved in my life thus far, and there have been many.
I don't feel it's boastful to say as much, as the greatness lies not in any brilliance on my part, but in the simple tastes of the ingredients as they fuse in the heat. That's home cooking for you.
I do rather love the way these little macaroni and cheeses, with their pixie-penne, look like they've been made with artificially colored, cheap squeezy-cheese or out of a box, when in fact their exotic glow comes courtesy of a sweet potato.
Preheat the oven to 400 degrees F. Put on a large-ish pan of water to boil, with the lid on to make it come to the boil faster.
Peel the sweet potatoes and cut them roughly into 1-inch pieces. When the water's boiling, add salt to taste, and then the sweet potato pieces, and cook them for about 10 minutes or until they are soft. Scoop them out of the water into a bowl—using a "spider" or slotted spoon—and lightly mash with a fork, without turning them into a purée. Don't get rid of this water, as you will need it to cook your pasta in later.
In another saucepan, gently melt the butter and add the flour, whisking to form a roux, then take the pan off the heat, slowly whisk in the milk and, when it's all combined and smooth, put back on the heat. Exchange your whisk for a wooden spoon, and continue to stir until your gently bubbling sauce has lost any floury taste and has thickened. Add the mustard and the 1/4 teaspoon of paprika. Season to taste, but do remember that you will be adding Cheddar and salty feta later, so underdo it for now.
Cook the pennette in the sweet-potato water, starting to check 2 minutes earlier than packet instructions dictate, as you want to make sure it doesn't lose its bite entirely. Drain (reserving some of the pasta cooking water first) and then add the pennette to the mashed sweet potato, and fold in to combine; the heat of the pasta will make the mash easier to mix in.
Add the feta cheese to the sweet potato and pasta mixture, crumbling it in so that it is easier to disperse evenly, then fold in the white sauce, adding the 1 1/4 cups grated Cheddar as you go. Add some of the pasta cooking water, should you feel it needs loosening up at all.
Check for seasoning again, then, when you're happy, spoon the brightly sauced macaroni cheese into 4 small ovenproof dishes of approximately 1 1/2 to 1 3/4 capacity (or 1 large rectangular dish measuring approx. 12 x 8 x 2 deep and 6 1/2 cup capacity). Sprinkle the remaining Cheddar over each one, dust with the remaining ¼ teaspoon of paprika, then shred the sage leaves and scatter the skinny green ribbons over the top, too.
Put the dishes on a baking tray, pop into the oven and bake for 20 minutes (or, if you're making this in a larger dish, bake for 30-35 minutes), by which time they will be piping hot and bubbling, and begging you to eat them.
MAKE AHEAD NOTE
The macaroni cheese can be made 1 day ahead. When the pasta has cooked, reserve 7 tablespoons of the cooking water and add this to the white sauce (it may look a little thin but the pasta absorbs the sauce as it cools). Transfer to the ovenproof dishes (without the sage topping). Once cool, cover and refrigerate within 2 hours of making. Sprinkle with the Cheddar, paprika and sage just before baking and cook for an extra 5 - 10 minutes, checking that the macaroni and cheese is piping hot in the center before serving.