1. Wash and dry the rhubarb, remove the ends, and cut into 2.8 inches (7cm) long pieces. If the rhubarb stalks are quite thick, cut them in half or thirds lengthwise. Add to a bowl and mix with 1 tablespoon of the xylitol and let sit for about 20 minutes, stirring occasionally.

2. Preheat the oven to 350°F (180°C). Lightly grease a 9 inch (23cm) pie dish and set aside. Separate the eggs and whisk the whites with a pinch of salt. Once the egg whites start to foam up, gradually add about 1 tablespoon of the xylitol and continue whisking until stiff peaks form. Set aside.

3. Add the soft butter together with the remaining xylitol to a large bowl and mix for about 2-3 minutes until light and fluffy. Add the lemon zest and vanilla extract and mix until well combined. Add the egg yolks, one at a time, and mix well after each addition. When all egg yolks are added mix for another 2-3 minutes until the mixture is very fluffy. Combine the ground almonds with the flour and baking powder and add to the bowl – mix until just combined. Add the whisked egg whites in several batches and fold in gently to keep as much volume as possible. Transfer the batter to the prepared pie dish and smooth out the top. Place the rhubarb pieces in a checkerboard pattern on top of the batter. When you reach the edges of the pie dish trim the rhubarb pieces so they fit snuggly and the top of the cake is covered completely with one even layer of rhubarb. Sprinkle a few sliced almonds on top and bake for 40-45 minutes until the cake has set completely. Turn off the oven, open the door a bit and leave the cake in the oven for another 5 minutes, then take out and let cool down on a wire rack. Serve with some whipped cream or vanilla sauce.