Start by making the almond filling. In a bowl, mix together unsalted butter, granulated sugar, almond meal, 2 large eggs, and vanilla or almond extract until well combined. Set aside.
Slice the croissants in half horizontally.
Generously spread the prepared almond filling on the sliced sides of the croissants.
Place the filled croissants, filling side up, into a buttered baking dish.
Sprinkle sliced almonds over the top of the filled croissants.
In a separate bowl, whisk together the custard ingredients: 1 cup milk, 3 eggs, 1/4 cup sugar, and 1 tsp salt until well combined.
Pour the custard mixture evenly over the filled croissants in the baking dish, ensuring all the croissants are soaked in the custard.
Cover the baking dish with aluminum foil.
Preheat the oven to 375 degrees Fahrenheit (190 degrees Celsius).
Bake the covered croissant French toast in the preheated oven for about 20-25 minutes.
Remove the foil and continue baking for an additional 5-10 minutes or until the top is golden brown and the custard is set. Keep an eye on it to prevent over-browning.
Once done, take the croissant French toast out of the oven and let it cool for a few minutes.
Serve warm, and if desired, top with additional sliced almonds or powdered sugar. Enjoy your delightful Almond Croissant French Toast!