Preheat oven to 375°F (190°C). Pour macaroni into a large bowl, cover with hot water, stir in salt. Let soak for 30 minutes.
Melt 2 tbsp butter. Pulse toasted white bread in a food processor until coarse crumbs form. Add melted butter and pulse to combine. Set aside.
Melt remaining 5 tbsp butter in a large saucepan over medium heat. Add flour, cook, stirring constantly, until light golden blonde. Whisk in evaporated milk and whole milk VERY slowly, creating a smooth, thick sauce. Stir in ground mustard. Bring to a simmer, then remove from heat.
Stir in grated cheddar and melting cheese until completely melted and smooth. Season to taste.
Whisk about 1 cup of cheese sauce with the beaten eggs in a separate bowl to temper them. Gradually add this egg mixture back into the main sauce, stirring constantly. Add drained macaroni and stir to combine everything.
Pour the mac and cheese mixture into a greased rectangular or square baking dish. Sprinkle the prepared breadcrumbs evenly over the top.
Bake for 25-30 minutes, or until bubbly and the top is golden brown and crispy. Let stand for 5-10 minutes before serving.