Grease a 9-inch coated cast-iron skillet with butter. Arrange the sliced croissants in the skillet, overlapping them.
In a medium bowl, whisk together eggs, granulated sugar, brown sugar, cinnamon, nutmeg, and salt. Once well combined, add milk, heavy cream, bourbon (if using), and vanilla extract. Whisk until well blended.
Pour the custard mixture over the croissants, then tuck raspberries and chocolate between each slice.
Cover with foil and refrigerate overnight or for at least 6 hours.
When ready to bake, preheat the oven to 350˚F. Create the cinnamon sugar topping by mixing sugar and cinnamon in a small bowl. Sprinkle it over the croissants, cover with foil, and bake for 30 minutes. Uncover and bake for an additional 30 minutes or until the center is set.
Let it cool for about 30 minutes, then sprinkle toasted hazelnuts and more fresh raspberries on top before serving.