Prepare the yogurt by combining the milk and milk powder (if using) in a saucepan and heat over a moderate heat. Remove from the heat when the milk starts to steam and bubbles appear around the edge.
Meanwhile, warm a large mixing bowl in a microwave then pour the heated milk into it. Allow it to cool until a thermometer reads 46°C. Whisk in the live yogurt, then cover the bowl with clingfilm and wrap the bowl in a clean towel. Place in an airing cupboard and leave for 6-8 hours (or overnight depending on when made).
Meanwhile prepare the rhubarb compote; place the rhubarb, sugar and water in a saucepan and bring to a simmer over a medium heat. Cover and lower the heat, cooking the rhubarb gently for 8-10 minutes until completely soft. Remove from the heat and stir a few times to combine everything.
Transfer to a clean bowl, cover and chill until needed. Check the yogurt to see if it has thickened and looks set; if so, pour into a clean, airtight jar and chill until ready to serve.
When ready to serve, spoon the yogurt into serving bowls. Top with the rhubarb compote and the toasted oats on top of that.