Pat the pineapple rings dry on kitchen paper and arrange them on 2 slices of panettone. Scatter with the marshmallows and crushed macadamia nuts and sprinkle with the vanilla sugar. Top with the remaining 2 slices of panettone.
Toast in a sandwich grill for 2–3 minutes, or according to the manufacturer's instructions, until the bread is golden and the marshmallows are beginning to melt. Slice each sandwich into small rectangles and dust with icing sugar. Serve immediately.