Preheat oven to 350 degrees.

Cook penne according to package instructions until al dente.  Keep in mind that the pasta will continue to cook in the oven so you don't want to overcook them.  Add about a tablespoon of kosher salt to the water when cooking the noodles, it will make a difference in the final taste of this dish.  It sounds like a lot of salt but much of the salt is gone when you drain the pasta.  Drain and set aside.

In a large saucepan over medium heat, melt butter.  Once butter has melted slowly add the flour to make a roux, whisking constantly until just turning a golden color, about 5 minutes.  Congratulations, you have now made a Bechamel sauce.  Slowly add the milk and cream, once again whisking constantly.  raise heat and bring to a simmer.  Continue to cook until the sauce thickens and coats the back of a spoon, about 3-5 minutes more.

Season with salt and pepper to taste, add mustard powder.  Gradually add the cheese, about a cup at a time, stirring until each cup of cheese has melted and is smooth.

Pour the penne into the cheese sauce and stir well until all of the noodles are well coated.

Butter the muffin or doughnut tin.  Mini tins don't work well because of the size of the noodle.  The doughnut tin makes fun wheel shapes and the muffin tin turns out the trusty muffin shape that's so easy to grab and go.  Fill to the rim, the doughnut tin will raise above the top of the pan.  Bake for 20 minutes until the muffins are set.

In a medium saute pan, heat oil over medium heat, add spinach, cover and cook for 2 minutes.  Uncover, increase heat to medium high and continue to cook for another minute until all of the spinach has wilted.  This is an optional step, especially if you are serving kids that don't like to see green in their mac and cheese.