Fill a large pot with water (about 3 quarts) and plenty of salt (about 2 tablespoons, this helps season the pasta, you won’t be eating all of it!) and bring to a boil. Add pasta and cook until al dente. Reserve 1 ½ cups of the pasta water. Drain pasta in a colander and set aside.
To make the garlicky nuts (optional): line a small plate with paper towel. In a large skillet over medium-low heat, add 1 tablespoon of butter. Once melted, add the nuts. Cook, stirring often, until they just start to turn golden brown, about 3-4 minutes. Add the garlic and stir for 30 seconds. Remove from heat and pour the nuts on the prepared plate, sprinkle with salt, and toss to coat.
Wipe out the skillet, return to medium heat, and add the olive oil and 1 tablespoon butter. Once melted, add the black pepper and chili flakes and let cook for 30 seconds. Stir in the kale, brussels sprouts, salt, and garlic. Cook until kale and brussels have softened and liquid has evaporated, stirring occasionally, about 7-8 minutes.
Add the drained pasta to the skillet along with 1 cup of the reserved pasta cooking water and remaining 1 tablespoon butter. Use tongs to toss and turn the pasta to coat. Cook until most of the water has evaporated and a glossy sauce forms, about 3-5 minutes. Stir in the parmesan towards the end.
Serve topped with more parmesan, garlicky nuts (or simply toasted nuts), and black pepper.