Cook the pasta according to package directions until al dente. Drain and set aside.
In a saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1-2 minutes.
Gradually whisk in the milk and bring to a simmer. Cook, stirring frequently, until thickened to a creamy sauce consistency.
Remove from heat and stir in the gochujang and mustard powder, if using. Add the cheddar cheese in batches, stirring constantly until melted and smooth after each addition.
Add the cooked pasta to the cheese sauce and stir to coat. Transfer to a baking dish.
Top with panko breadcrumbs and scallions. Bake at 400°F for 15-20 minutes until bubbly and golden brown.
Let rest 5 minutes before serving. Enjoy!