Reserve 75 ml of milk and bring the rest to a boil. Stir the cornstarch and sugar into the boiling milk until dissolved.
Soak the dried lingonberries in rum.
Coarsely chop the baking chocolate. Remove the milk from heat and stir in the chopped chocolate. Mix until smooth.
Pour about half of the mixture into a pudding mold and place over an ice bath to chill quickly. Once the pudding has started to solidify, sprinkle about 3/4 of the soaked lingonberries over the surface. Top with the remaining pudding mixture, cover with plastic wrap and refrigerate overnight.
Carefully turn the pudding out onto a serving platter. Sprinkle with the remaining soaked lingonberries.
Dust with dark cocoa powder, cut into slices and serve.