In a pot, combine rice, coconut milk, water, salt, and sugar; mix and cover to cook until done. Fluff cooked rice and keep warm.
Mix rice vinegar and sugar in a shallow bowl; add sliced cucumbers and toss to coat. Let sit while preparing salmon.
Preheat oven to broil on high (550°F). Place salmon cubes in a large bowl; add oil, soy sauce, brown sugar, ground ginger, garlic powder, sesame seeds, and optional chili flakes. Toss to combine.
Spread salmon evenly on a rimmed baking sheet. Broil for 6 to 8 minutes until browned and cooked through.
Meanwhile, mix mayonnaise, sriracha, and lime juice in a small bowl to make spicy mayo.
Assemble bowls by dividing rice, topping with pickled cucumber, cooked salmon, sliced avocado, and chopped chives. Drizzle spicy mayo on top and season with salt if needed.