Preheat oven to 200 ℉.
In a large mixing bowl, whisk together the room temperature eggs, sugar, and cinnamon. Whisk vigorously, so that the cinnamon mixes well.
Add half and half, heavy cream, and pure vanilla extract. Whisk until well combined. Transfer mixture to a shallow dish.
In a heavy-duty skillet, melt the butter over medium low heat.
Place 1 -2 croissants into the custard mixture and allow it to soak for 30 seconds. Flip, and soak for another 30 seconds. Using a fork, hold the soaked croissant over the dish for a few seconds to let the custard drain any excess before placing it in the skillet.
Place the croissant halves into the hot skillet, making sure not to overcrowd the skillet. Cook for 2-3 minutes, until golden brown. Flip, and cook for another 2-3 minutes.
Remove from skillet and place on a baking sheet in the preheated oven, to keep warm while you repeat the process and cook the remaining croissants. Add additional butter while cooking, if needed.
Serve warm and top with fresh fruit, powdered sugar, maple syrup, or whipped cream.