Grease a box cake tin, length 30 cm, and dust with flour. Preheat the oven to 175 ° C top / bottom heat.
Beat the butter, powdered sugar, salt and rum until frothy. Beat in the eggs one by one. Mix the flour and baking powder, sieve and fold in alternately with the egg liqueur. Spread half of the dough into the pan.
Melt the mint chocolate in a water bath and stir into the remaining batter. Place the dark dough on the light dough, spread evenly and use a fork to spiral through the cake dough.
Put the tin in the oven and bake the cake for about 55 minutes. Take the cake out of the oven and leave it in the tin for another 10 minutes. Then place on a wire rack and let cool down.
Melt the chocolate with 1 teaspoon of oil in a water bath and coat the surface of the cake with it.