Preheat the oven to 350°F/180°C and line a 9x5-inch loaf pan with baking paper. Grease the baking paper and set it aside.
Start with the cinnamon swirl: In a small mixing bowl combine the Erythritol with the cinnamon and set aside.
In another bowl, whisk together the eggs, Erythritol, sunflower oil, almond milk, and vanilla. Set aside.
In a third bowl, combine the almond flour, coconut flour, a pinch of salt, and baking powder.
Finally, combine the egg mixture and flour mixture together into a batter.
Pour half the amount of the batter onto the prepared loaf pan.
Sprinkle half the reserved cinnamon swirl mixture, add the remaining batter and sprinkle the remaining cinnamon swirl mixture on top.
Swirl a skewer into the batter to create a marble effect.
Cover with a piece of aluminum foil and bake the cake for 40-45 minutes, or until a toothpick inserted in the center of the cake comes out clean.
Allow to cool in the pan before removing and slicing. Serve at room temperature.