Devices that confine and process single electrons represent an important scaling limit of electronics(1,2). Such devices have been realized in a variety of materials and exhibit remarkable electronic, optical and spintronic properties(3-5). Here, we use an atomic force microscope tip to reversibly 'sketch' single-electron transistors by controlling a metal-insulator transition at the interface of two oxides(6-8). In these devices, single electrons tunnel resonantly between source and drain electrodes through a conducting oxide island with a diameter of similar to 1.5 nm. We demonstrate control over the number of electrons on the island using bottom-and side-gate electrodes, and observe hysteresis in electron occupation that is attributed to ferroelectricity within the oxide heterostructure. These single-electron devices may find use as ultradense non-volatile memories, nanoscale hybrid piezoelectric and charge sensors, as well as building blocks in quantum information processing and simulation platforms.