Bring 3 cups water to a boil in a medium saucepan. Stir in rice, bring to a boil, cover and reduce to a simmer over low heat; cook until rice is tender, 16–20 minutes. Fluff rice with a fork and stir in sesame seeds; transfer rice to a sheet pan and chill in refrigerator. 

Preheat oven to 450ºF (232ºC) with racks in top and bottom thirds. Place a baking sheet on bottom rack while oven preheats.

Halve each chicken breast horizontally into two thin breast halves, cover chicken halves with plastic wrap and pound to an even ¼-inch thickness with a meat tenderizer; pat chicken dry then season lightly with salt.

Toss broccolini with 2 teaspoons oil and season with salt and pepper. 

Carefully remove baking sheet from oven and coat with nonstick spray. Arrange chicken on half of the baking sheet and broccolini on the other half in a single layer. Spoon 1 tablespoon gochujang sauce on top of each chicken piece and brush to coat.

Roast chicken and broccolini on bottom rack for 6 minutes. 

Remove sheet from oven, flip chicken pieces and spoon 1 tablespoon of gochujang sauce on top of  each chicken piece; brush to coat. Return sheet to oven on bottom rack and roast 4 minutes. Transfer sheet to top rack, turn oven to broil and cook for 2 minutes until chicken is cooked through and an instant-read thermometer inserted in the centers registers 165ºF (73ºC) and broccoli starts to char. 

Meanwhile, heat a large nonstick skillet (10-inch) over medium-high for 3 minutes. Add remaining 2 teaspoons grapeseed oil and heat until shimmering. Add rice in an even layer, pat down into a cake, and cook until lightly golden brown, 5–7 minutes. Gently flip cake with a spatula (it’s OK if it breaks into a few pieces) and cook 5 minutes more until crispy and golden brown. 

Serve chicken and broccolini over crispy rice, spooning any pan juices over chicken. Drizzle each serving with toasted sesame oil and sprinkle with scallions and sesame seeds. Serve with additional gochujang sauce on the side.