For the panini, begin by making your own chocolate-hazelnut spread, similar to the well-known Italian treat Nutella. You may prefer to use brioche, whole wheat, cinnamon raisin, white or even rye bread. Consider adding a sprinkling of sweetened shredded coconut, chopped dried fruit or a tiny bit of chopped chocolate. But don't fill the panini too full, or it will leak.
Requires a panini maker and serves 4 (with leftover chocolate-hazelnut spread)
Toast the hazelnuts on a baking sheet for about 12 minutes, or until golden brown and fragrant. Pulse the hazelnuts in a food processor, scraping down the sides of the bowl occasionally, for 2 to 3 minutes, until the nuts turn into a paste (or butter). Add the chocolate and pulse until the chocolate is finely ground. Leave the mixture in the processor.
Bring the cream, sugar and salt to a boil in a small saucepan over medium heat, stirring to dissolve the sugar. With the motor running, pour the cream mixture through the feed tube and process, scraping down the sides of the bowl occasionally, for 3 to 5 minutes, until the mixture is very smooth. Transfer to a bowl and stir in the vanilla. (You will have 1 cup chocolate-hazelnut spread that will last for at least 1 month.) Refrigerate, tightly covered, until ready to use.
Preheat a panini maker or waffle iron as the manufacturer directs.
Spread one side of each bread slice with butter. Arrange 4 slices, buttered side down, on a sheet of waxed paper. Spread each slice with 1 tablespoon of the chocolate hazelnut spread. Cover with the remaining slices, buttered-side up.
When the panini maker is ready, add the sandwiches. (If the panini maker is not large enough, bake 1 panini at a time.) Cover and cook for 2 minutes, or until the panini are golden brown; do not lift cover during baking. Repeat with remaining panini. Serve hot.