Chop the croissants into bite-sized pieces.
Combine the remaining ingredients in a large bowl, whisking until well combined.
Stir in the croissant pieces.
Spray the wells of a muffin tin or add cupcake papers.
Scoop the croissants into the prepared wells until just below the rim, making sure the liquid is evenly distributed between the wells.
Refrigerate, uncovered, overnight, up to 24-hours.
Pre-heat oven to 325°F.
Bake for 45-55 minutes or until the muffins have puffed and no longer look wet (if they are removed too early, they will deflate while cooling, but will still be fine to eat).
Serve warm with syrup or powdered sugar.