There are two possible solutions to this problem: 1) Fry the oliebollen at a lower temperature. A very high frying temperature can cause the outside to burn before the heat has time to penetrate within. A good temperature is 175-185°C (347-365°F). 2) Lower the amount of sugar and/or milk in the recipe. Both of these accelerate browning through the Maillard reaction. Adding less of either of them will slow down browning, giving you more time to fry the oliebol!