Heat oven to 190C. Cut plums in half,discard stones,and arrange on a baking tray lined with baking paper. Scatter with two tablespoons castor sugar,drizzle with liqueur and bake for 20 minutes or until soft and juicy.
Whisk the eggs with the remaining sugar and ½ teaspoon vanilla until smooth. Whisk in milk and pour into a flat container. Soak the bread in batter,turning once,until soaked.
Heat half the butter in a non-stick frypan and fry four slices of soaked bread on medium for two minutes on each side until golden,then repeat with remaining butter and soaked bread.
Transfer to a baking tray and bake for 10 minutes until fluffed up.
To make the vanilla yoghurt,whisk the yoghurt,one tablespoon icing sugar and remaining vanilla until smooth.
Top two overlapping slices of toast with three or four cut-side-up plums,dust with icing sugar and scatter with mint. Serve with vanilla yoghurt.