Meanwhile, bubble 140g blackberry jam with 450g blackberries for 6-8 minutes in a shallow pan until they start to soften, then turn off the heat.
When the brioche has a few minutes left in the oven, whisk 4 egg whites in a very clean bowl until they form peaks.
Gradually add 100g golden caster sugar and continue to whisk until peaky again. When the brioche is golden and puffed up, spoon over the blackberries and top with egg white.
Scatter each with a little sugar, then bake for 15 minutes until golden. Serve warm.