Whisk 3 Tbsp. gochujang, 2 Tbsp. soy sauce, 1 Tbsp. bouillon paste, 1 Tbsp. sugar, 2 tsp. fish sauce, and 1 cup water in a medium high-sided skillet to combine.
Slice the tofu in half lengthwise, then cut crosswise into 6 sections to create 12 squares. Arrange in pan in a single layer.
Scatter the onion, garlic, and 1 Tbsp. butter on top. Set pan over high heat and bring to a boil. Cook, spooning sauce over tofu occasionally, until sauce is reduced by about a third, 5–7 minutes.
Remove pan from heat. Add 1 tsp. toasted sesame oil and remaining 3 Tbsp. butter, and stir gently until butter is melted and incorporated into sauce.
Crush seasoned toasted seaweed snacks into large shards and scatter over tofu. Serve with cooked rice alongside.
