Coat the mushrooms in the olive oil, season them with salt and pepper and lay them on baking sheet, making sure that they are in one flat layer so the mushrooms crisp instead of steam. Bake for 10 to 15 minutes, then flip the mushrooms over and bake until brown and crispy, about 10 minutes. Set them aside to cool.
Bring a large pot of heavily salted water to a boil. Cook the shells according to the instructions on the package.
In a high-powered blender or food processor, grind the Parmesan into a fine powder.
Add half of the butter into a large saucepan over a medium heat. Add in the miso and sugar and stir until combine into a beautiful, saucy paste. Slowly stream in the milk, whisking continuously to make sure the miso and butter emulsify with the milk. Add 1 piece of the remaining butter, whisking until melted. Once melted, add another piece of butter. Repeat with the last pieces of butter.
Drain the pasta and reserve 1 cup of the pasta water. Add the cooked pasta into the saucepan with your miso sauce. Add a splash of the pasta water, followed by a small handful of the Parmesan, alternating back and forth between the two to make sure the cheese has melted and the sauce is thickening.
Plate the mac and cheese and top it with more Parmesan, the crispy mushrooms and the chopped chives. Enjoy!