1. Preheat the oven to 220ºC
2. Place butter and sugar in a saucepan on low heat and stir together until butter is melted and sugar dissolves
3. Place rhubarb in the pan and top with the puff pastry, tucking edges in around the sides of the pan
4. Brush pastry with egg wash and place pan in the oven for 15-20 minutes (or until golden and puffed)
5. Once cooked, carefully flip tarte onto serving plate and serve with vegan ice cream