Combine yeast, 1 tbsp sugar and milk in a cup and let sit for 5 minutes while preparing batter. Cover with a paper towel to speed up the yeast rising.
Melt butter.
Combine butter, ½ cup granulated sugar, egg yolks and salt in a mixing bowl and mix together until creamy.
Add yeast mixture and continue mixing.
Add vanilla sugar and rum and continue mixing.
Add the flour and juice from an orange to the mixing bowl and mix together with a wooden spoon until combined.
Add raisins (if desired).
When all blended, let the batter sit for ½ hour in the same bowl covered with a paper towel or kitchen cloth.
Use a spatula and mix again, leave the batter for another 20 minutes covered.
Grease bundt pan.
Pour batter into the pan.
Bake 375F for 25 minutes. Check with a toothpick if the cake is ready. It is better to be undercooked than overcooked.
Remove from the pan. Once cool enough to the touch, remove the pan and let it cool completely on the wire rack.
Sprinkle with icing sugar when cool.