Preheat your oven to 350°F. Grease a 9x9-inch baking pan and line it with parchment paper for easy removal.
In a large bowl, whisk together the almond flour, coconut flour, erythritol, baking powder, and salt.
In another bowl, beat the eggs, then add the melted butter, almond milk, and vanilla extract. Mix until well combined.
Pour the wet ingredients into the dry ingredients and mix until you have a smooth batter. Pour half the batter into your prepared pan.
Make the cinnamon swirl by mixing together the softened butter, brown sugar substitute, and cinnamon until it forms a paste. Drop spoonfuls over the batter in the pan, then use a knife to swirl it through the batter.
Pour the remaining batter on top and add more cinnamon swirl, creating beautiful layers. Swirl again gently with a knife.
Bake for 25-30 minutes, until a toothpick inserted in the center comes out clean. Let it cool completely in the pan.
While the cake cools, make the frosting by beating together the cream cheese and butter until fluffy. Add the powdered erythritol and vanilla, then beat in the heavy cream one tablespoon at a time until you reach your desired consistency.
Once the cake is completely cool, spread the cream cheese frosting over the top. Cut into squares and serve. This dessert tastes even better the next day after the flavors have melded together!