Preheat oven to 350°F. Spray a 13×9 baking dish with cooking spray (or butter dish).
Slice panettone into 1/2-inch thick slices (6 to 8 slices). Set aside one large slice. Cut the slices down the center and place them in a single layer, slightly overlapping in the prepared pan. You may not have to use all of the slices.
In a large mixing bowl, whisk together the eggs, sugar, and salt. Add the milk and heavy cream and whisk until smooth. Pour the mixture over the panettone slices, and press them down with your hands so that all the pieces are soaked with the milk mixture. Cover the pan with aluminum foil and bake for 45 minutes to 1 hour, or until the bread has absorbed the custard, is puffed, and custard is not runny. Allow to cool until warm, about 10 minutes.
Place the reserved panettone slice on a work surface and roll flat with a rolling pin. Use a star-shaped cookie cutter to stamp shapes from the flattened slice. If you have other leftover slices, use them for star cut-outs also. Dust the star shapes with confectioners’ sugar. Top baked French toast with star cut-outs. Dust with more powdered sugar.