Preheat your oven to 325°F and prepare a large baking dish to serve as a water bath.
In a small saucepan over medium heat, simmer the maple syrup for 3-4 minutes until it darkens slightly and bubbles vigorously to create a clean caramel.
Immediately pour the hot maple syrup into an 18-ounce oven-safe ramekin, tilting the dish to coat the entire bottom surface evenly.
In a medium mixing bowl, whisk together the whole eggs, liquid egg whites, Greek yogurt, vanilla extract, sea salt, and cinnamon until the mixture is completely smooth and homogenous.
Strain the custard mixture through a fine-mesh sieve directly into the prepared ramekin to ensure a perfectly velvety texture.
Place the ramekin inside the larger baking dish and fill the outer dish with hot water until it reaches halfway up the sides of the ramekin.
Bake for 45 to 55 minutes, or until the edges are set and the center has a slight, uniform jiggle when moved.
Remove the ramekin from the water bath and let it cool to room temperature on a wire rack.
Refrigerate the flan for at least 4 hours, or ideally overnight, to allow the custard to fully set and the caramel to liquefy.
To serve, run a thin knife around the inner edge of the ramekin and invert the flan onto a rimmed plate, allowing the maple caramel to flow over the top.