Heat your oven to 275°F. Arrange sweet potatoes on a large, foil-lined baking sheet. Rub each with 1 teaspoon olive oil, 1/2 teaspoon salt (which will make a quite salty skin, use less if desired) and 1/4 teaspoon pepper until well coated.
Bake until very soft inside and caramelized on the bottom, about 2 1/2 hours.
Heat your broiler and run the potatoes underneath it — mine is fairly weak, and this took 5 to 10 minutes, but check in regularly, a more robust one might do it in 1 to 2 minutes — until lightly charred on top.
Let potatoes cool 10 minutes, then gently crush potatoes with your hands to expose the flesh.
Split sweet potatoes, brush insides with melted butter. Sprinkle with sugar and a dash of cinnamon.