Preheat your oven to 325°F. Grease a 9x5-inch loaf pan and line the bottom and two long sides with a strip of parchment paper, leaving an overhang for easy removal.
In a bowl, whisk together the almond flour, coconut flour, baking powder, and salt. In a large bowl, beat the softened butter, cream cheese, and erythritol together on medium-high speed for 3 to 4 minutes until very light and fluffy.
Add the eggs one at a time, beating well after each. Mix in the vanilla extract. Fold in the flour mixture on low speed until just combined and smooth.
Transfer about one-third of the batter to a separate bowl. Sift the cocoa powder over it and add the 1 tablespoon of melted butter. Fold together until the chocolate batter is evenly dark and smooth.
Spoon alternating dollops of the vanilla and chocolate batter into the prepared loaf pan. Once all the batter is in, drag a toothpick or skewer through the batter in a zigzag pattern to create beautiful marble swirls.
Do not over-swirl - three or four passes is perfect for a dramatic pattern.
Bake for 55 to 65 minutes until the top is golden and a toothpick inserted in the center comes out clean. If the top browns too quickly, tent it loosely with foil after 40 minutes.
Let the pound cake cool in the pan for 15 minutes, then use the parchment overhang to lift it out. Cool completely on a wire rack before slicing into thick, gorgeous marble slices.
