Add the Greek yogurt, heavy cream, and sweetener to a medium mixing bowl. Start blending on low speed so the ingredients combine smoothly without splashing. Once everything begins to come together, increase to medium speed and blend until the mixture looks thick, creamy, and completely smooth, with no streaks of yogurt or cream left.

Add the cinnamon, vanilla extract, salt, and protein powder to the bowl. Blend again, starting slowly, then increase speed until the mousse is fully combined and silky. Scrape down the sides of the bowl with a spatula once or twice to make sure no protein powder or cinnamon is hiding on the edges. The mixture should look fluffy and uniform, not grainy or lumpy.

Taste the mousse using a clean spoon and adjust as needed. If you want it sweeter, add a little more sweetener. If you prefer a stronger cinnamon flavor, sprinkle in a small pinch at a time and blend briefly after each addition. This is the step where you customize the mousse to your exact taste, so take your time.

Spoon or pour the finished mousse into individual serving glasses or bowls. Smooth the tops with the back of a spoon, cover, and refrigerate for at least 30 minutes to let the mousse set and thicken slightly. Serve chilled for the best creamy texture and flavor.