Bring a large pot of water to a boil. Add the spaghetti and cook according to package instructions. Reserve 1 cup of the cooking water. Drain the spaghetti and return to its pot.

Melt 4 tablespoons of butter in a skillet over medium-low. Add the garlic and season with salt. Cook, stirring occasionally, until the garlic starts to soften but not brown, 1 to 3 minutes.

Stir in the gochujang, honey, and vinegar. Bring to a simmer over medium-high. Cook, stirring constantly, until the mixture reduces significantly, about 3 to 4 minutes. Remove from heat.

Transfer the sauce to the pot with the spaghetti. Add the remaining 2 tablespoons butter. Stir until melted. Add splashes of the reserved cooking water to thin the sauce. Season with salt and pepper. Top with cilantro or scallions if using and serve immediately.