How to Make It
Remove the ribs and stems from the kale leaves. Rinse, and then add the leaves to a pot with 2 tablespoons of water over medium heat. Cover and simmer until kale is tender but can still hold its shape, about 4 minutes. Drain excess water from the braised kale leaves, and add to the bowl of a food processor. Pulse until finely chopped, and transfer to a small bowl.
In the food processor, combine the ricotta cheese, Parmigiano-Reggiano cheese, salt and pepper, and blend to a smooth consistency. Add the kale and pulse until kale is evenly distributed into mixture.
Spread a small amount of the semolina flour in a 2-inch-deep, 9x9 casserole dish or pan. With a tablespoon measure, portion out the ricotta mixture and roll into small donut hole sized balls. Set each ball into the semolina flour and roll to coat, repeat with each ball. Pour remaining flour over the prepared gnudi, ensuring they are entirely covered with the semolina. Cover with plastic wrap and refrigerate for 24 hours.
To prepare gnudi, have 6 tablespoons of butter in a large nonstick skillet ready on the stove.
Bring a large pot of generously salted water to a boil over high heat. Meanwhile, remove the gnudi from the semolina flour, and gently dust each one slightly to remove any excess semolina. Transfer the pieces all to one large plate. With a slotted spoon, drop the gnudi into the boiling water and cook for no more than 1 minute. Warm the skillet with the butter on medium high heat.
Remove the gnudi from the water and set into the pan of butter. Pan fry the pasta for 2 minutes on each side while the butter starts to turn golden brown, being careful not to burn. One minute before taking off heat, add the sage. Transfer gnudi to a serving platter, along with the browned butter and sage leaves. Season with salt and serve immediately.