Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally until cooked through but firm to the bite, about 8 minutes; drain.
Melt 3 tablespoons butter in a large saucepan over medium heat; stir in shallots and cook and stir until soft, about 3 minutes. Stir flour into shallot mixture and cook for 1 minute; whisk in milk. Bring milk mixture to a gentle boil, whisking constantly.
Set aside a large pinch of the Cheddar cheese, Gruyere cheese, and a few cubes of the Brie cheese to use for topping. Stir Dijon mustard and remaining Cheddar cheese, Gruyere cheese, and Brie cheese into milk mixture; remove from heat and stir until all cheese is melted, about 2 minutes; season with salt and pepper.
Stir macaroni and lobster meat into cheese mixture; pour into a baking dish.
Whisk panko bread crumbs, Parmesan cheese, and reserved Cheddar and Gruyere cheese together in a bowl; sprinkle over the top of lobster mixture. Cut remaining 1 tablespoon butter and reserved Brie cheese into tiny pieces and scatter evenly over crumb mixture.
Bake in the preheated oven until bubbly and golden, about 20 minutes.
I blended two recipes to make this one. I had originally meant to add cream, but found with the Brie it was creamy and rich enough. If you want, you can add 1 cup of heavy cream to the shallot and flour mixture in Step 3. I think it would be very rich if you do.