Preheat oven to 400°F. Coat a 9×13 inch baking dish with cooking spray and set aside.

Bring a large covered pot of salted water to a boil over medium-high heat. Add pasta to boiling water and cook until just al dente (the pasta will still be firm but not crunchy). Drain pasta and set aside.

In a large pot over medium-low heat, melt the butter. Sprinkle the flour over the butter, whisking to combine and make a roux. (It will look like a paste.) Continue cooking, stirring constantly with a wooden spoon, until the roux is golden brown, about 4 minutes. Slowly add the milk and cream, whisking constantly to incorporate and create a béchamel sauce. Stir in the nutmeg. Increase the heat to medium-high and bring the sauce to a low boil, whisking constantly.

Reduce to a simmer, whisking occasionally, and cook until the béchamel sauce is thick and coats the back of a spoon, about 4-5 minutes. Stir in the salt, pepper, and mustard powder. Add the cheeses, 1 cup at a time, whisking until each addition is completely melted before adding more. Remove from heat. Stir the drained pasta into the sauce and toss to coat. Fold in the lobster meat.

Transfer macaroni mixture to the prepared baking dish and place on a rimmed baking sheet covered with foil for easy clean up. Stir the butter into the panko bread crumbs. Sprinkle panko topping evenly over pasta.

Bake until golden and bubbling, approximately 20-25 minutes. Allow to cool 15 minutes before serving.

Sprinkle with parsley, if desired.