In a large bowl, whisk the eggs, milk, caster sugar and a pinch of salt together until well combined.
Heat half the butter in a large non-stick frying pan over a medium heat. Coat 2 bread slices in the egg mixture and place in the pan. Sprinkle each slice with a quarter of the chocolate. Soak 2 more bread slices in the egg and place on top of the chocolate. Press down gently. Cook for 3 minutes each side, until golden.
Repeat with the remaining ingredients. Slice into fingers and serve dusted with icing sugar.