Preheat your oven to 325°F. Grease a standard 9-inch x 5-inch (or slightly smaller) loaf pan, and set it aside.

In the bowl of a stand mixer fitted with the paddle attachment (or a large bowl with a handheld mixer), cream the butter until light and fluffy. Add the sugar, eggs, and vanilla, mixing well after each addition. In a separate medium-size bowl, whisk together the flour blend, xanthan gum, baking powder, baking soda and salt. Add the flour mixture in 3 parts to the butter mixture, alternating with the milk, mixing until just combined after each addition until all of the flour mixture and milk have been added. The batter should be light and smooth. Scrape half of the batter into a separate, medium-size bowl and set it aside.

In a small bowl, place the cocoa powder and warm water and mix together until smooth. Scrape the cocoa mixture into the mixing bowl and mix with the remaining half of the vanilla batter until just combined.

Marble the separate batters together in the prepared pan in the following manner: Begin by placing one portion of about 2 tablespoons of vanilla batter to the bottom center of the loaf pan (using spring-loaded ice cream scoops or a combination of spoons and spatulas). Shake the pan from side to side gently to spread the batter a bit. Place an equal portion of the chocolate batter in the center of the vanilla batter, and again shake the pan from side to side gently. Place a second portion of vanilla batter in the center of the chocolate batter, shake the pan again gently, and repeat with the chocolate batter, alternating until all the batter is in the pan.

Place the pan in the center of the preheated oven and bake until a toothpick inserted in the center comes out clean (about 45 minutes). Remove from the oven and allow the cake, still in the pan, to cool on a wire rack for at least 20 minutes. Carefully remove the cake from the pan, and place it on the wire rack to cool completely. Slice and serve.