Pour 100 ml of boiling water over Glühfix. Leave to stand for 5 minutes, remove the bag. Let the Glühfix cool down. Meanwhile peel the grapefruit so that the white skin is completely removed.
Cut grapefruits into 9-10 slices. Beat butter, salt, sugar, cinnamon and vanilla sugar with the whisk of the hand mixer until foamy. Add the eggs bit by bit. Mix flour and baking powder, add and stir in.
Stir the milk and half of the Glühfix infusion into the dough. Grease a Savarin mould (1250 ml content) well, line it with the grapefruit slices and sprinkle with brown sugar. Spread the dough evenly on top and bake in a preheated oven (electric range: 175 °C/ gas: level 2) for approx. 35 minutes.
In the meantime, warm the jam and then pass it through a sieve. Take the cake out of the oven, place it on a grid and let it rest for about 5 minutes. Sprinkle with remaining Glühfix. Turn the cake out of the tin, spread with jam and let it cool down.
Dust the cake with icing sugar before serving. Decorate with cinnamon stars as desired.