In a blender or food processor, combine gochujang, soy sauce, garlic, ginger, brown sugar, rice vinegar, and water. Blend until smooth.
Heat the sesame oil in a small saucepan over low heat. Add the blended mixture and whisk until smooth.
Bring the mixture to a simmer and cook for about 5 minutes, or until it thickens slightly.
Remove from heat and stir in black pepper and crushed red pepper flakes (if using).
Let the samjang cool to room temperature before serving.
