For the poached rhubarb, add the sugar and water to a medium saucepan and bring to a rolling boil, stirring occasionally. Once boiling and the sugar has dissolved, add the rhubarb chunks and cook for 2 minutes at a rolling boil. Then remove from the heat and leave to sit for at least 15 minutes, if not longer for them to cool in the syrup.
Preheat the oven to 180c fan/200c/400F.
Add the eggs, flour, milk, caster sugar, vanilla extract and a pinch of salt to a blender and blitz to form a smooth batter. Alternatively, whisk together in a large mixing bowl until completely combined, smooth and lump-free (you may wish to sieve the flour first to help with this).
Add the butter to a 25.5cm (10-inch) cast iron pan with fairly deep sides or 23cm (9-inch) baking dish. Place in the oven to melt the butter.
Once the butter has melted and is bubbling hot (don’t leave it too long or it will burn and become bitter), pour the batter into the pan/baking dish and carefully return to the oven (just as you would for a Yorkshire pudding). Bake for around 20-25 minutes - or until puffy, golden and a skewer inserted into the centre comes out clean. It should be all puffed up - a bit like a soufflé (it will naturally deflate and become more custardy - don’t worry, this is normal. It hasn’t gone wrong!).
Whilst baking, whip the mascarpone cream. Add the double cream and icing sugar to a large bowl and whisk until very loose, soft peaks. Then add the mascarpone, at room temperature, and vanilla. Keep whisking until soft peaks and thick, but not stiff.
Remove from the oven and dust with icing sugar. Then, if desired, top with poached rhubarb and dollops of whipped mascarpone cream. Or cut into slices and top each slice. Enjoy!