Preheat the oven to 350°F. Line a loaf pan (8 x 5 x 3.5 inches) with parchment paper.
Separate the egg whites from the egg yolks, setting aside 3 yolks for the chocolate layer and 3 yolks for the vanilla layer. Place the egg whites in a large mixing bowl, and using a hand mixer, whip the egg whites until stiff peaks form. Set aside.
For the Chocolate Layer:
Set up a double boiler by placing a heatproof bowl on top of a saucepan filled with about 1 inch of water. Make sure that the bowl is not touching the water and is suspended above the water line by the rim of the pan. To the bowl, add the chocolate chunks and coconut oil, and whisk to combine until melted and smooth. Remove the bowl from the pan and set aside.
In a separate mixing bowl, combine the egg yolks, erythritol, almond milk, vanilla extract and salt, and whisk until all ingredients are well incorporated. Slowly fold in the melted chocolate. Once the chocolate in mixed in, gently fold in half of the whipped egg whites.
Add the cocoa powder, almond flour, and baking powder and slowly mix until smooth. Be careful not to over mix and deflate the eggs.
For the Vanilla Layer:
In a mixing bowl, mix together the almond flour, erythritol, coconut flour, and baking soda.
In a separate large mixing bowl, combing the remaining 3 egg yolks, almond milk, coconut oil and vanilla extract. Gently fold in the remaining whipped egg white.
Add the dry ingredients to the wet ingredients and mix until all of the ingredients are well incorporated
Putting it all together:
Spoon 1/3 of the chocolate mixture into the bottom of the prepared loaf pan. Top with half of the vanilla mixture. Layer 1/3 of the chocolate mixture on top of the vanilla layer. Top with the remaining vanilla mixture. Finish by layering on the last 1/3 of the chocolate mixture.
Using a knife, gently swirl the layers together a bit to make a marbling effect, but without completely mixing them together.
Bake for 45 minutes or until a toothpick comes out mostly clean, but with a little bit of cake stuck to it. It will continue to firm up as it cools.
Place the loaf pan on a cooling rack and let cool before slicing.