Place 4 medium croissants in the basket or tray of your air fryer (if using a traditional convection oven, place them on a wire rack inserted into a baking sheet). Air fry (or bake) at 350°F for 7 minutes or until the edges feel crisp and the croissant is heated through.
While the croissants are air-frying, whisk together 2 cups powdered sugar, 2 Tablespoons water, and ¼ teaspoon vanilla extract in a medium bowl.
When the croissants are done cooking, immediately transfer them to a wire rack over a baking sheet. While the croissants are still warm, brush, drizzle, or dip them into the prepared glaze, allowing any excess to drip onto the baking sheet below. Try to coat all sides of the croissant with the glaze. Place the glazed croissants back onto the wire rack, and let them set for 5-10 minutes, in which time the glaze will harden.
If desired, mix up to 1 Tablespoon cocoa powder into any leftover glaze, and pipe or drizzle this thicker chocolate glaze over the croissants (alternatively, use melted chocolate if desired). Enjoy the glazed croissants still slightly warm or at room temperature.