Set an oven rack about 6 inches from the heat source and preheat the oven to 220°C.
Bring a large pot of salted water to a boil. Add the macaroni and cook until it is al dente, about 6 minutes. Strain, reserving 1 3/4 cups of the pasta water. Set aside.
Heat a large saucepan over medium-high heat. Add 3 tablespoons of the olive oil and the shiitake mushrooms and cook, stirring constantly, until the mushrooms are lightly browned in spots and softened, 4 to 6 minutes. Lower the heat to medium and add the kale, garlic and 1/2 cup of water, scraping up any browned bits at the bottom of the pan with the back of a wooden spoon. Add 3/4 teaspoon salt and 1/4 teaspoon pepper and stir to combine. Cook until the kale is wilted but still crisp-tender, about 3 minutes. Remove the vegetables to a bowl and set aside.
Carefully wipe out the saucepan (or give it a quick rinse if it is really dirty). Add the single cream and cayenne and bring to a simmer over medium heat. Cook until reduced to 1 1/2 cups, about 15 minutes. Add the cream cheese and stir until melted. Whisk in the Cheddar and Gruyere until all the cheese is melted and the sauce is smooth.
Add the macaroni and reserved pasta water to the saucepan and stir to combine. (The mixture will look very loose, but the pasta will suck up a lot of the sauce as it bakes.) Gently fold in the reserved vegetables. Season to taste with additional salt if needed. Pour into a 13-by-9-inch baking dish. Toss the bread crumbs, remaining 1/4 cup olive oil and a large pinch of salt and pepper in a medium bowl and sprinkle evenly over the macaroni and cheese. Bake until the cheese is bubbly and the bread crumbs are lightly browned and crispy, about 10 minutes.
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