Preheat your oven to 325°F. Grease well a standard 10-cup bundt pan (See Recipe Notes), and set it aside.

In the bowl of a stand mixer fitted with the paddle attachment (or a large bowl with a handheld mixer), beat the butter, sugar, and vanilla until light and fluffy (about 5 minutes).

Add the eggs, and mix until well-combined. The mixture should be smooth and silky.

In a separate medium-size bowl or large measuring cup, whisk together the flour blend, xanthan gum, cornstarch, baking powder, and salt.

Add the flour mixture in 3 parts to the butter mixture, alternating with the milk, and mixing until just combined after each addition until all the flour mixture and milk have been added. The batter should be light and smooth.

Scrape about 1/3 of the batter into a separate, medium-size bowl and set it aside.

In a small, heat safe bowl, place the cocoa powder. Pour the hot water over the cocoa powder, and whisk until very smooth.

Scrape the cocoa mixture into the mixing bowl and mix with the remaining third of the vanilla batter until smooth. Add the melted chocolate, and whisk again until smooth.

Scrape half of the vanilla batter into the prepared bundt pan and spread into an even layer on the bottom of the pan. Be sure to spread the batter into all the corners of your pan.

Place the pan in the center of the preheated oven and bake until a toothpick inserted in the center comes out clean, rotating once halfway through baking (about 1 hour).

Remove from the oven and allow the cake, still in the pan, to cool on a wire rack for at least 20 minutes.

With clean hands, pull the edges away from the side of the bundt pan. Slide a silicone, nylon, or wooden spatula or knife along the center hole to help release it from the cake.

Place a wire rack on top of the pan, and invert the pan and wire rack so the rack is on the countertop. Tap the outside of the pan a few times with a wooden spoon.

Remove the pan to reveal the bundt. Allow the cake to continue to cool before slicing and serving.