Grease a Guglhupf mould with butter and dust with flour. Pour in the sand cake batter and bake in the oven for around 45 minutes at 170 °C. Remove from the oven and allow the Gugelhupf to cool in its mould for 20 minutes. Meanwhile, make the simple syrup, allow it to cool, and mix in the rum. Next, turn over the Guglhupf mould and lift it off the cake. Pour the rum and simple syrup mixture onto the cake slowly so that it can be absorbed.