Heat the oven to 425F. Halve the sweet potatoes lengthwise, rub the cut faces with a little oil, and lay them cut side down on a bare metal sheet pan. Roast 35 to 45 minutes, until the halves have collapsed and a knife meets no resistance.

Flip the halves cut side up and leave them for 10 minutes so the steam escapes. Scoop the flesh into a large bowl, discard the skins, and mash it smooth. You want about 1 ½ cups.

Put a dry skillet over medium-high heat and add the corn straight from frozen. Cook 2 to 3 minutes, stirring, until the water has cooked off and a few kernels show brown spots. Tip onto a plate to cool.

Stir the egg, milk, green onions, cilantro, cumin, smoked paprika, salt, black pepper and cayenne into the mashed sweet potato until uniform.

Add the flour, cornmeal and baking powder and stir just until no dry streaks remain. Fold in the toasted corn last so the kernels stay whole.

Cover and rest 15 minutes at room temperature. The batter thickens as the cornmeal hydrates. It should mound on a spoon and then slowly relax. Add cornmeal a tablespoon at a time if it runs, or milk a tablespoon at a time if it is stiff.

Stir the yogurt, grated garlic, lime juice, cilantro and a pinch of salt together in a small bowl and set it aside to sit.

Pour vegetable oil into a heavy skillet to a depth of ¼ inch and heat to 350F over medium. A half teaspoon of batter should sizzle steadily and rise within two seconds.

Drop heaping tablespoons of batter into the oil, three or four at a time, and nudge each into a rough 3 inch round with the back of the spoon. Do not press them flat. Cook 3 minutes, until the edges are deep golden and set, then flip once and cook 2 to 3 minutes more.

Move the fritters to a wire rack set over a sheet pan, never onto paper towels, and salt them lightly while hot. Hold them on the rack in a 200F oven while you fry the rest. Serve warm with the garlic lime yogurt dip.