Preheat oven to 450°.
Use about 1 tablespoon of vegetable oil to coat the bottom of a large roasting pan or oven-safe skillet.
Stir milk, flour, salt and baking powder together in small bowl. Dip cauliflower florets in mixture to coat, shaking off any excess batter. Arrange in a single layer in roasting pan or skillet. Bake 10 minutes.
While cauliflower bakes, combine soy sauce, maple syrup, rice vinegar, sesame oil, ginger, garlic and sriracha in a small saucepan. Place over medium heat and bring to a simmer. Allow to simmer for 10 minutes. Stir cornstarch and water together in a small container until cornstarch is fully dissolved. Add to soy sauce mixture and stir to blend. Remove from heat.
Remove cauliflower from oven and flip. Lightly brush florets with remaining tablespoon of oil. Return to oven and bake 5 minutes more, until they begin to sizzle and lightly crisp.
Remove from oven and pour sauce over florets. Return to oven and bake 3-4 minutes more, just until sauce becomes very sticky and coasts florets.
Serve over rice and sprinkle with sesame seeds and scallions.