2. Prepare the miso barbeque sauce. Combine the miso, mirin, sake, sugar, egg yolk, dashi, and lemon zest in the top of a double boiler and whisk until smooth. Cook the sauce over the double boiler until thick and creamy, 3 to 5 minutes. Do not overcook, or the sauce will curdle.
3. Set up your grill for direct grilling and preheat to high. Brush and oil the grill grate. Skewer the tofu pieces through the narrow end, 2 skewers per piece. Arrange the resulting kebabs on the grate. Grill the tofu until sizzling and well browned, 4 to 6 minutes per side. (Protect the skewers with strips of foil if they start to burn.) Carefully spoon the sauce over the tofu pieces after you've turned them. (The tofu goes only on the top side.)
4. Transfer the tofu to plates or a platter and top with any remaining sauce. Sprinkle with the sesame seeds and serve at once.