Pat the pineapple rings dry on paper towels and arrange them on 2 slices of panettone. Scatter over 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.