In a large kettle, bring 4 quarts water and 2 T salt to a rolling boil. Add macaroni and continue to boil, stirring occasionally, for 9 minutes. Drain pasta but do not rinse.
In the same kettle, melt butter over med-hi heat. Whisk in flour, ½ tsp kosher salt, black pepper, cayenne, garlic salt and vanilla. Continue to cook, continually whisking, for 2 minutes or until the mixture is smooth.
Slowly pour in the milk and half & half. Cook, whisking continually for 8-10 minutes until the mixture is thickened. You may need to reduce the heat to med after 4 minutes of cooking if the mixture is at a hard boil. You want to allow it to reach a gentle, slow boil. Continual whisking is important to eliminate lumps.
Remove from heat. Stir in 2 (8 oz pkg) shredded medium cheddar. Continue stirring until cheese is melted.
Mix the cooked pasta, cheese sauce nd raw broccoli florets together. Pour into a 13x9 casserole dish that has been sprayed with Pam.
Top with 1 (8 oz) pkg shredded sharp cheddar and ¼ c breadcrumbs.
Bake in preheated 350 oven for 25 minutes. Cool 5 minutes before serving.