Put 100g/3½oz of the raspberries in a small bowl and sprinkle with the raspberry liqueur if using and 2 tablespoons of the caster sugar. Crush very lightly, then spoon this over the brioche layer.
If it looks cheap and nasty, it probably is. But don’t begrudge paying for a well-made liqueur; a little goes a long way. Being spirit-based, liqueurs have a long shelf life and, generally ...