Preheat oven to 200ºC. In a saucepan, add chopped rhubarb, Natvia, vanilla and water.
Cook over a medium to low heat, stirring regularly, until rhubarb begins to break down (approximately five minutes).
Add in Natvia raspberry jam and stir through. Continue to cook until mixture is soft and pulpy. Remove from heat and set aside to cool for 15 minutes.
Slice puff pastry dough into four even squares. Set pastry in a diamond direction and spoon two tablespoons of rhubarb and raspberry filling into the centre. Take two opposite corners and press together. Repeat with remaining pastry.
Line a tray with baking paper and place Danishes. Brush lightly with whisked egg. Bake for 15-20 minutes until pastry is golden.