Put the blackberries, sugar and 25ml (1fl oz) of water into a saucepan. Heat gently until all the sugar has dissolved and the blackberries start to release their juices. Set aside.
Mix the egg, milk and vanilla in a shallow bowl. Dip the brioche into the mixture and leave to soak for a minute or two. Turn the slices over so the bread is well coated and leave for a minute or two more.
Heat a frying pan over a medium heat. Add the butter and swirl around the pan. Cook the bread for 2-3 minutes on each side until golden all over.
Serve warm with the blackberry compote spooned over top. Add a dollop of yogurt, crème fraîche or vanilla ice cream if you like.