Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan and set aside.
In a large mixing bowl, combine the almond flour, coconut flour, erythritol, baking powder, and salt.
Add the softened butter to the dry ingredients and mix until well incorporated.
Beat in the eggs, one at a time, ensuring each is fully integrated before adding the next. Stir in the almond milk and vanilla extract until the batter is smooth.
In a small bowl, mix together the erythritol and cinnamon for the cinnamon swirl.
Pour half of the cake batter into the prepared pan. Sprinkle the cinnamon swirl mixture evenly over the batter.
Carefully spread the remaining batter on top of the cinnamon layer.
For the streusel topping, in a separate bowl, combine the almond flour, erythritol, and cinnamon. Cut in the cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs.
Sprinkle the streusel topping evenly over the cake batter.
Bake for 40 minutes or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.