Preheat oven to 180°C/160°C fan forced. Grease a 20cm (base measurement), 4cm-deep square ovenproof dish. Cut croissants horizontally in half. Spread bases with butter; top with 16 pieces of the chocolate (4 pieces each). Replace croissant tops; cut each croissant sandwich in half crosswise. Transfer to prepared dish in a single layer.
Lightly whisk eggs in a jug. Add essence, caster sugar, cream and milk; whisk until combined. Pour mixture over croissants. Stand for 10 minutes, turning and pressing to submerge croissants in egg mixture.
Sprinkle top with frozen raspberries. Break remaining chocolate pieces in half; sprinkle on top. Bake for 35 minutes; cover with foil. Bake for 10 minutes more or until set at centre. Stand in dish for 5 minutes. Dust with icing sugar.