1. Wash and dry the rhubarb, cut it into small pieces, and mix with some mashed raspberries (optional) and vanilla sugar in a small bowl. Set aside for about 1 hour, so it can marinate a bit – mix occasionally.

2. For the streusels add the flour and sugar to a bowl and mix to combine. Add the cold butter in small pieces and mix everything with your fingers until you get different sizes of streusels. Place in the fridge until needed.

3. Preheat the oven to 350°F (180°C). Line a 9×5 inches (23x13cm) loaf pan with a piece of baking parchment and grease lightly. Set aside.

4. Add the butter and sugar (or xylitol) to a large bowl and mix on high speed for about 4-5 minutes until very light and fluffy. Add the eggs one after another and mix well after each addition – butter and eggs will only combine well if they had the same temperature. Mix the flour, baking powder, and salt. Add together with the sour cream and vanilla extract to the bowl and mix on low until just combined. Sprinkle the flour over the marinated rhubarb and mix to combine, then add the rolled oats and mix in as well. Add the rhubarb to the bowl with the batter and fold in. Pour about half of the batter into the prepared loaf pan and smooth out. Sprinkle about half of the streusels on top and cover with the remaining batter. Smooth out the top. Mix the remaining streusels with the rolled oats and sprinkle on top of the batter.

5. Bake the cake for about 65-70 minutes or until a wooden skewer inserted into the center of the cake comes out clean. Remove from the oven and let cool down in the loaf pan for some time, then lift out of the pan with the parchment paper and let cool down completely on a wire rack.