Boil the 16 oz elbow macaroni in well-salted water for 1 minute less than package directions, about 7 to 8 minutes. Drain and set aside without rinsing.

Melt the 4 tbsp butter in a large heavy pot over medium heat, add the 4 tbsp flour, and whisk constantly for 90 seconds to cook out the raw flour taste.

Pour in the 3 cups whole milk and 1 cup heavy cream in a slow stream while whisking. Cook over medium heat for 5 to 6 minutes, whisking often, until the sauce thickens enough to coat the back of a spoon.

Reduce heat to low. Stir in the 1 tsp mustard powder, 1/2 tsp garlic powder, 1/2 tsp onion powder, 3/4 tsp kosher salt, and 1/4 tsp white pepper. Add the 2 cups white cheddar and 1 cup yellow cheddar in handfuls, stirring each addition fully before adding the next.

Fold the drained macaroni into the cheese sauce over low heat for 2 minutes, then ladle into bowls and finish with a pinch of extra white cheddar and a crack of black pepper.