Preheat the oven to 400ºF. Arrange the potatoes on a rimmed baking sheet and rub with 1 tablespoon of the olive oil. Bake the potatoes until tender when pierced with a knife, about 40 minutes. Set aside to cool.
While the potatoes bake, place the kale, broccoli, and water in a medium pot set over medium-high heat. Bring to a rapid simmer and then reduce heat to low, cover, and cook until the vegetables are bright green and the water is almost entirely evaporated, about 5 minutes. Transfer the steamed vegetables to a large mixing bowl, leaving any excess water behind. Add the scallions and garlic powder.
When the potatoes are cool enough to handle, cut them in half lengthwise. Using a small spoon or melon baller, scoop out the inside of each potato, leaving a ¼-inch thick border around the edge. Add the scooped out potato flesh and remaining 2 tablespoons olive oil to the bowl with the vegetables. Use a potato masher to work the mixture into a chunky mash. Season to taste with salt and pepper.
Divide the filling evenly among the hollowed out potato halves. Arrange in a baking dish and cover. Return to the oven for 10-15 minutes to heat through.
To Make Ahead: The stuffed potatoes can be refrigerated for up to 1 day or frozen for up to 1 month before baking. To reheat from the refrigerator, bring the dish to room temperature. Bake, covered, at 400ºF for 20 minutes. To reheat from frozen, thaw 24 hours in the refrigerator. Bring the dish to room temperature and bake, covered, at 400ºF for 30-35 minutes.