Preheat the oven to 400°F. Generously butter a 1-1/2-quart baking dish.
Mix the flour, salt, paprika, mustard, and cayenne pepper together; set aside.
Melt the butter in a skillet, add the green pepper and onion, and cook until soft. Stir in the flour mixture and cook, stirring and smoothing, for 2 or 3 minutes. Add the milk, stirring constantly, and bring it to the boiling point. Stir in the corn and egg yolk.
Spoon into the baking dish and sprinkle with the crumbs. Bake for 25 minutes until the crumbs are brown.