Preheat your oven to 350°F (175°C) and prepare a small baking pan by greasing it lightly or lining it with parchment paper.

In a mixing bowl, combine protein powder, sweetener, baking powder, and cinnamon. Whisk until thoroughly blended.

Crack in the eggs and pour in the almond milk, then mix until smooth and free of lumps.

Pour half of the batter into the prepared pan and smooth the surface.

Sprinkle a generous layer of cinnamon on top and add the remaining batter over it.

Use a knife to swirl the batter gently for a marbled effect.

Bake for approximately 12 minutes, until a toothpick comes out clean.

Let the cake cool in the pan for a few minutes before slicing.