Cut butter into pieces. Knead flour, cocoa, butter, sugar, sour cream, rum and egg with the dough hooks of the hand mixer, then work into a smooth dough with your hands. Wrap the dough in foil and chill for about 30 minutes
Roll out the dough on a floured work surface to a thickness of approx. 2 mm. Cut into long narrow rectangles (13 x 2 cm) with a serrated dough wheel. Cut an approx. 7 cm long slit in the middle of each strip. Pull one end of the strip through the slit to a skipper's knot
Heat the oil in a pot (approx. 160 °C). Fry the apron cake in portions for about 3 minutes. Take out and drain on kitchen paper. Dust the apron cake with icing sugar and serve lukewarm or cold
Waiting time approx. 30 minutes