Preheat the oven to Gas Mark 3, 170°C, fan 150°C. In a shallow bowl, beat the egg, milk and brown sugar. Scrape the seeds from the halved vanilla pod with the tip of a knife (reserve the empty pod) and add to the mixture.
Cut the slice of bread into 4 triangles and coat in the egg mixture. Set aside.
Place the rhubarb in a small ovenproof dish, pop in the scraped vanilla pod, and scatter with the caster sugar and 1tbsp water. Cover with foil and roast for about 20 minutes, until tender but still holding its shape.
Preheat a griddle or frying pan over a medium heat, add the butter and the soaked bread, drained of any excess liquid. Griddle or fry for 2 minutes or so on each side, until golden brown. Serve with the roasted rhubarb.