Melt 2 tablespoons butter in a small saucepan over medium-high heat.
Whisk in 2 tablespoons flour and cook, whisking, about 1½ minutes until the raw flour taste is gone and the roux is very light blond; do not brown.
Slowly add 1 cup whole milk, about 1/4 cup at a time, whisking after each addition to prevent lumps.
Add 1/4 cup heavy cream and continue whisking until the sauce comes to a gentle boil, then reduce to a simmer for 1–2 minutes, stirring occasionally.
Remove the pan from the heat and whisk in 2 cups grated medium-sharp cheddar until fully melted, then season with 1/4 teaspoon kosher salt.
Meanwhile, combine 2 tablespoons panko breadcrumbs with 2 tablespoons melted butter in a small bowl and set aside.
Preheat the oven to the broil setting (or 500°F if no broiler) and position a rack so the top of the skillet will be 4–6 inches from the heat source.
Cook 8 ounces dry macaroni according to package directions until al dente, then drain well in a colander.
Return the drained pasta to the saucepan with the cheese sauce and fold gently with a rubber spatula to combine.
Stir in the additional 1/2 cup grated cheddar and 1/2 cup grated Monterey Jack until incorporated.
Transfer the macaroni and cheese to a small cast-iron skillet or oven-safe ramekin.
Evenly sprinkle the buttered panko over the top.
Broil until the cheese is melted and the breadcrumbs turn golden brown, watching carefully to avoid burning (a minute or two).
Remove from the oven and serve immediately.
