1 large head of cauliflower, leaves removed, stem trimmed
1/2 tsp. (or more) kosher salt
4 Tbsp. unsalted butter
1/4 cup vinegar-based hot sauce (such as Frank’s)
1 Tbsp. ketchup
1 Tbsp. soy sauce
1/2 cup mayonnaise
2 Tbsp. white miso
1 Tbsp. fresh lemon juice
1/2 tsp. freshly ground black pepper
2 scallions, thinly sliced
Prepare a grill for medium-high heat. Sprinkle cauliflower all over with salt in a large microwave-safe bowl. Cover with plastic wrap, pierce plastic a few times with a knife to vent, and microwave on high until a paring knife easily slides into stem, about 5 minutes. Let cool slightly. (Alternatively, set a steamer basket in a large pot filled with about 1" salted water. Cover pot and bring water to a boil. Add cauliflower, cover, and steam until a sharp paring knife easily slides into stem, about 5 minutes. Transfer to a plate and let cool slightly.)
Heat butter, hot sauce, ketchup, and soy sauce in a small saucepan on grill, stirring occasionally, until butter is melted, about 2 minutes. Brush cauliflower all over with sauce and grill, covered, 10 minutes. Turn cauliflower over, brush with sauce, and grill, covered, 10 minutes. Continue to grill, brushing and turning every 10 minutes and reheating sauce as needed, until cauliflower is lightly charred on all sides and fork-tender, 25–30 minutes. The sauce should be used up by now, but if not, brush any remaining sauce over. Transfer cauliflower to a plate and let cool slightly.
Whisk mayonnaise, miso, lemon juice, and pepper in a medium bowl until smooth. Spread on a plate. Set cauliflower on top and scatter scallions over.