1. Preheat oven to 350°F. Cook pasta until al dente, following directions on the package. Drain well.
2. Heat the milk in a small saucepan, do not boil. In a large pot, melt 6 tbsp. of the butter and add the flour. Cook over low heat for 2 minutes, stirring constantly with a whisk. Continue whisking, add the hot milk and cook for 1 to 2 minutes, until thickened and smooth.
3. Remove from heat, add the gruyere, cheddar, Old Bay seasoning, and a pinch of salt and pepper. Stir until the cheese melts. Gently fold in the cooked pasta and shrimp.
4. Pour mixture into a 9x13 baking dish. Melt the remaining 2 tbsp. of butter, combine with the panko and Old Bay. Sprinkle on top. Bake for 30-35 minutes, until the sauce is bubbly and the pasta is browned on top.