Mix room temperature butter and the dark rye bread to a lumpy mass. Put baking paper in a baking form with removable edges. Put the bottom mixture in the baking form, and press the mixture evenly on the bottom of the form so it becomes a cake bottom.
Place the bottom in the fridge while you prepare the filling to the cake.
Place the gelatin sheets in cold water for ten minutes. While the gelatin sheets are in the water continue with the filling.
If you use fresh shrimps it is time to peel them now. Mix the shrimps together with the 300 gr of the cream fraiche, the mayonnaise and the cream cheese.
Cut the red onion and the dill in small pieces, and pour them down in the shrimp mixture.
Take up the gelatin sheets from the water and squeeze them firmly. Pour the remaining cream fraiche in a casserole and heat it up slowly on low heat. Add the gelatin sheets and let them melt slowly whole stirring.
Pour the creme fraiche and gelatin mixture in the shrimp mixture and mix it all together. Take out the baking form with the bottom and pour the filling on top of the bottom. Place the cake in the fridge for minimum 5 hours before serving.
Before serving the cake it is nice to decorate it with some dill, shrimps, lumpfish roe and a citron.