Cut the custard into 24 rectangles. In a large, deep nonstick skillet, heat 1/2 inch of vegetable oil until shimmering. Working with 6 pieces of custard at a time and keeping the rest refrigerated, dip the custard rectangles in the beaten eggs, then dredge in the flour. Fry the custard over moderate heat, turning once or twice, until golden and crisp, about 5 minutes. Transfer to a rack lined with paper towels and immediately sift cinnamon sugar over them. Repeat with the remaining pieces of custard. Serve warm or at room temperature.