Preheat the oven to 350°F (175°C). Grease or line a loaf pan with parchment paper.
Prepare the cinnamon swirl: In a small bowl, mix together the granulated erythritol and ground cinnamon for the swirl. Set aside.
Mix dry ingredients: In a large bowl, whisk together the almond flour, coconut flour, granulated erythritol, baking powder, ground cinnamon, and salt until well combined.
Add wet ingredients: Add the eggs, melted butter, unsweetened almond milk, and vanilla extract to the dry ingredients. Mix until smooth and thoroughly combined.
Layer the batter and cinnamon swirl: Pour half of the batter into the prepared loaf pan. Sprinkle half of the cinnamon swirl mixture evenly over the batter. Repeat with the remaining batter and cinnamon swirl mixture.
Swirl the cinnamon: Use a knife to gently swirl the cinnamon mixture into the batter, creating a marbled effect.
Bake: Transfer the loaf pan to the preheated oven and bake for 40-45 minutes, or until a toothpick inserted into the center comes out clean.
Cool and slice: Allow the bread to cool in the pan for 10-15 minutes before transferring it to a wire rack to cool completely. Once cooled, slice and serve.