Pour the wine into a saucepan and heat it up. Put the mulled wine spice in a tea filter, then add it to the wine and let it steep for 10-15 minutes. Remove the spice and let the mulled wine cool down.
Preheat the oven to 180 °C top/bottom heat and grease a Bundt cake tin.
Put the butter together with the sugar, Tonka sugar and salt in a bowl and beat until white and foamy using a hand mixer for 5-7 minutes. Now add eggs one at a time, stirring thoroughly for 30-60 seconds after each egg.
Place the flour, nuts and baking powder in a bowl and mix together. Now add to the butter-sugar mixture. Also add the mulled wine and mix everything thoroughly.
Pour the dough into the prepared baking pan and bake on the middle rack for 35-45 minutes (test with a chopstick!).
Remove the finished cake from the oven, let it cool for 10 minutes and then turn it out onto a wire rack.
To make the icing, mix the powdered sugar with the mulled wine and pour over the cake. Add the chopped hazelnuts on top and let the icing set.
