Add the lingonberries to a pan and gently heat until they burst. This should take no longer than a couple of minutes. Mash with a fork.
Place the berries in the fridge to cool while you prepare the other ingredients.
Pre-heat the oven to 175℃ (350℉).
Grease and flour a bundt or tube pan. See here for more information. Melt the butter and set to one side.
In a bowl or mixer, add the eggs and sugar. Whisk until light and fluffy (and the colour becomes paler).
Add the cooled melted butter and gräddfil (see note 1). Continue whisking until everything is combined.
Mix all of the remaining dry ingredients together and slowly add to the wet. Stop mixing when everything pulls together.
Pour around ⅔ of your batter into your pan.
Take the cooled lingonberries and dollop over the top of your batter, and swirl it in with a knife.
Pour over the remaining batter and level it off. Bang the pan on the counter a couple of times to get rid of any air bubbles.
Bake in the middle of the oven for around 35 minutes (start checking at around 25 minutes). An inserted skewer or toothpick will come out clean when the cake is ready.
Remove from the oven and allow the cake to sit in the pan for 10 minutes.
When ready, remove the cake for the pan and allow to cool completely before icing.