Preheat the oven to 400 degrees . In a large pot of boiling, salted water, cook the buckwheat, undisturbed, until tender, about 15 minutes; drain.
Meanwhile, in a medium, ovenproof nonstick skillet, heat the olive oil over medium heat; add the zucchini, two-thirds of the garlic and 1/2 teaspoon salt and cook until the zucchini is brown around the edges, about 10 minutes. Stir in the buckwheat and cook for 2 minutes; season with 1/4 teaspoon salt.
In a large bowl, combine the eggs, 2 tablespoons basil and 1/4 teaspoon each salt and pepper. Add to the skillet and cook, scrambling, until half-cooked. Transfer to the oven and bake until the frittata is set, 8 to 10 minutes; cut into wedges.
Meanwhile, in a small bowl, stir together the sour cream with the remaining garlic and 1 tablespoon basil. Add a dollop to each wedge and sprinkle with the sliced basil.