Filling: Peel the rhubarb and cut it into 1 cm long pieces. Melt the fat on a flat cake mould or on a clean cast-iron pan. Add the brown sugar and rhubarb cubes.
Pastry: Beat the fat and sugar together until creamy. Add the eggs one at a time. Mix the baking powder and vanilla sugar with the flour. Add them and lemon juice (and grated lemon skin). Pour the pastry over the rhubarb filling.Bake at 175°C for about 45 minutes. Turn the tart upside down on a tray and serve it either warm or cold.