Pour vegetable oil in a small saucepan over medium-high heat. Using a candy thermometer or instant read thermometer, heat oil until it reaches 350℉.
Place the corn starch, beaten eggs and breadcrumbs in three separate shallow bowls.
Add a pinch of salt to the breadcrumbs.
Remove chilled custard from the refrigerator. Slice the custard into 2-inch cubes in the dish.
Take one cube and dip it in the cornstarch to coat it. Shake off excess starch and dunk it in the egg, turning it to cover it so there's no white showing. Roll the cube in breadcrumbs.
Using tongs, drop the cube in the hot oil. Let cook until golden brown, about 30-90 seconds.
Using a slotted spoon, remove the cube from the hot oil and place on a paper towel.
Sprinkle with cinnamon sugar. Let cool slightly and serve.
Repeat for remaining custard cubes.