Prepare the Roasted rhubarb, then cool and drain off the juices (the juice is good mixed into plain yogurt). Heat oven to 180C/fan 160C/gas 4. Line a 12-hole muffin tin with muffin paper cases. In a large bowl, beat together the sugar, oil, egg, orange zest and soured cream until there are no lumps and all are completely combined. Gently mix in the flour, then fold in the rhubarb.
Divide 3⁄4 of the batter evenly between the paper cases to generously fill. Using a spoon, press in the centre of each muffin to make a dip. Fill each dip with a small blob of chilled custard, then gently smooth the rest of the muffin mix over the custard. Bake for 25-30 mins or until risen, pale golden and oozing custard. Sprinkle each with a little caster sugar while still warm. Best eaten the day they are made.