Cook the kale for 5 minutes in a large pot of boiling salted water. Transfer to a colander with a slotted spoon (keep the pot of cooking water at a boil). Run the kale under cold water to stop the cooking and then drain and squeeze out any liquid. Coarsely chop.
Add the macaroni to the boiling kale-cooking water and cook according to package directions. Drain.
Meanwhile, combine the dried porcini with 1 cup warm water in a small bowl. Let stand until the mushrooms have softened, about 20 minutes. Lift the mushrooms from their soaking liquid with your fingers, leaving the grit behind. Line a fine-mesh sieve with paper towels, a paper coffee filter or cheesecloth. Pour the mushroom-soaking liquid through the sieve into a bowl. Reserve the liquid. Coarsely chop the mushrooms.
Preheat the oven to 350 degrees F.
Heat the oil over medium heat in a 5-quart Dutch oven or other large heavy-bottomed pot. Add the garlic and cook, stirring frequently, until tender, about 2 minutes. Add the porcini and cremini mushrooms, sage and thyme and cook, stirring occasionally, until the mushrooms have wilted and released their juices, about 5 minutes.
Stir in the flour and cook for 2 minutes. Add the mushroom-soaking liquid, milk, paprika and salt and cook, stirring occasionally, until the mixture has thickened, about 5 minutes. Remove from the heat and stir in the cheese until melted. Add the macaroni and kale and toss to coat. Transfer the mixture to a 9- by 13-inch glass baking dish or individual ramekins.
Melt the butter over medium heat in a small skillet Add the panko and toss to coat. Scatter the butter crumbs over the mac and cheese. Bake until the sauce is bubbling and the top is crunchy and golden brown, 30 minutes.
Cook's Notes: For variations on this recipe, swap in another green for the kale, such as escarole, or even cooked chopped broccoli.
Use Gruyere instead of Cheddar. For an extra kick, add 1/8 teaspoon cayenne pepper and 1 teaspoon Dijon mustard to the white sauce. Instead of buttered bread crumbs, top the casserole with crushed potato chips.
Instead of making a large mac and cheese, separate the mixture into two 9- by 9-inch pans and freeze one, unbaked and well wrapped, for a later date. You can bake the frozen mac and cheese straight from the freezer; it should take about 45 minutes.