Reduce heat to medium and add remaining 1 tsp. oil. Set a small handful of scallions aside for serving; add remaining scallions to skillet and cook, stirring often, until wilted and starting to brown, about 2 minutes. Add corn and cook, stirring, until kernels are well coated, about 2 minutes. Pour in mirin, 3 Tbsp. soy sauce, and 2 Tbsp. water and bring to a simmer. Nestle in tofu and cook until sauce reduces slightly, about 3 minutes. Swirl in butter and cook until sauce is glossy and coats tofu, about 2 minutes.