Preheat the Oven. Preheat your oven to 350°F. Grease a bundt pan generously with butter or cooking spray.
Prepare the Dry Ingredients. In a bowl, whisk together almond flour, coconut flour, baking powder, and salt. Set aside.
Cream the Butter and Sweetener. In a large mixing bowl, cream together the softened unsalted butter and Erythritol (or your preferred keto-friendly sweetener) until light and fluffy.
Add Eggs and Vanilla. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Incorporate the Dry Ingredients. Gradually add the dry ingredients to the wet ingredients, mixing well to combine.
Add Heavy Cream. Pour in the heavy cream and continue to mix until the batter is smooth and well combined.
Divide the Batter. Divide the batter into two equal portions in separate bowls.
Chocolate Swirl Mixture. In one bowl, add unsweetened cocoa powder to create the chocolate swirl mixture. Mix until well combined.
Create the Marble Effect. Spoon dollops of the vanilla and chocolate batters alternately into the prepared bundt pan. Use a knife or skewer to swirl the batters together, creating a marble effect.
Bake to Perfection. Bake in the preheated oven for 40-45 minutes or until a toothpick inserted into the center comes out clean.
Cooling. Allow the cake to cool in the pan for 10-15 minutes, then transfer it to a wire rack to cool completely.
Serve. Once the cake has cooled, slice and serve. Optionally, dust the top with powdered Erythritol or a drizzle of sugar-free chocolate sauce.