Grease a bundt cake pan well and sprinkle with breadcrumbs or flour. Preheat the oven to 180 degrees top / bottom heat.
Melt the chocolate with the butter in a double boiler and allow to cool. Beat egg whites with a pinch of salt until stiff. Sprinkle in 50 g of sugar and continue to beat firmly. Beat the egg yolks with the remaining 130 g of sugar until frothy. Stir in butter chocolate, rum and mineral water.
Mix the flour, baking powder and oatmeal and stir into the batter. Carefully fold in the egg whites. Dust the blueberries lightly with flour and fold into the batter. If frozen blueberries are used, do not defrost them beforehand! Pour the batter into the Gugelhupf pan and bake for about 45-50 minutes. Chopstick sample!