Whisk in cornstarch and cook, stirring, for 1 minute or until bubbling
Gradually whisk in milk, blending well and bring mixture to a boil, whisking constantly
Cook for about 3 minutes or until very thick
Remove from heat and whisk in mozzarella, Parmesan, and washed-rind cheese until well-blended
Pour into an 8-inch glass baking glass, smoothing the top
Cover and refrigerate for at least 1 hour, until chilled, or up to 1 day
Preheat oven to 375°F and butter a large baking sheet
In a bowl, mix together egg and water. Place breadcrumbs in another bowl.
Invert chilled cheese mixture onto a cutting board, cut into 16 squares
Dip each square first into the egg mixture; let excess drip off and coat it in the bread crumbs pressing lightly to help crumbs stick
Place the squares at lease 1" apart on the prepared baking sheet
Bake for 15 minutes or until the squares feel soft to the touch and the bottoms are golden brown. Flip over to show the golden side and serve.
Notes
Use 1 cup total of one or two varieties of cheese; just the mozzarella, and the Parmesan for a mild version or just the washed-rind cheese for a more pungent version