Preheat your oven to 350°F (175°C). Line an 8x4-inch loaf pan with parchment paper, leaving an overhang on the long sides.
In a large bowl, whisk together the almond flour, coconut flour, erythritol, baking powder, and salt. In a separate bowl, whisk the eggs, melted butter, heavy cream, and vanilla extract until smooth.Pour the wet ingredients into the dry and stir until a thick, smooth batter forms with no lumps.
In a small bowl, mix together the erythritol and cinnamon for the swirl filling. Set the softened butter aside nearby.
Pour half of the batter into the prepared loaf pan and spread it evenly. Dot half of the softened butter in small pieces across the top, then sprinkle half of the cinnamon mixture evenly over the batter.Pour the remaining batter on top and spread it out gently. Dot the rest of the softened butter on top and sprinkle the remaining cinnamon mixture over it.
Use a toothpick or thin skewer to drag through the batter in a zigzag pattern, creating a swirl effect. Don't over-mix, just a few passes.
Bake for 45 to 55 minutes, until the top is golden brown and a toothpick inserted in the center comes out clean. If the top is browning too fast, tent with foil after 35 minutes.
Let the bread cool in the pan for 15 minutes, then lift it out using the parchment overhang and transfer to a wire rack. Allow it to cool completely before slicing for the cleanest cuts.