In a large saucepan, melt butter over medium heat. Blend in flour and add bay leaf; cook, stirring, for 30 seconds. Pour in 1 cup milk, whisking constantly, until mixture comes to a boil and is very thick. Pour in rest of milk in a slow stream, whisking constantly, until sauce comes to a full boil and is smooth. Whisk in mustard.
Reduce heat to low; stir in cheese until melted. Remove bay leaf; season with salt and a dash of cayenne pepper to taste. Remove from heat.
Meanwhile, in a large pot of boiling salted water, cook macaroni for 8 minutes or until just tender. (do not over cook; pasta continues to cook in sauce) Drain well. Stir into cheese sauce until well coated.
Spoon into prepared casserole dish. In a bowl, toss 1 Tbsp. melted butter with bread crumbs; sprinkle over top. Bake in preheated oven for 25 minutes or until bubbly and top is lightly browned.