Step 1: Grease a 9x13 inch (23x33 cm) baking dish with some of the melted butter or non-stick spray. Preheat your oven to 375°F (190°C).
Step 2: Tear or slice the croissants into large, bite-sized pieces (about 1.5-2 inches). Arrange them in a single layer in the prepared baking dish.
Step 3: In a large bowl, whisk together the eggs, milk, heavy cream, granulated sugar, vanilla extract, and salt until well combined and smooth.
Step 4: Sprinkle about half (3/4 cup) of the chocolate chips evenly over the croissants in the baking dish.
Step 5: Slowly pour the egg mixture evenly over the croissants, making sure every piece gets saturated. Gently press down on any floating croissant pieces to help them absorb the liquid.
Step 6: Sprinkle the remaining chocolate chips over the top. Cover the baking dish tightly with plastic wrap and refrigerate for at least 4 hours, or ideally, overnight (12-18 hours).
Step 7: The next morning, remove the baking dish from the refrigerator and let it sit at room temperature for at least 30 minutes while the oven preheats to 375°F (190°C).
Step 8: Drizzle the remaining melted butter (about 2-3 tablespoons) over the top of the croissants.
Step 9: Bake uncovered for 35-40 minutes, or until the top is golden brown, puffed up, and the custard is set in the center (a knife inserted near the center should come out clean).
Step 10: Let the bake cool for 5-10 minutes before serving. Dust with powdered sugar, if desired, and serve warm.