Preheat your oven to 325°F. Grease very well a standard cast aluminum light-colored 12-cup bundt pan.
If your pan is at all scratched or if it's older, scatter 1 to 2 additional tablespoons of cocoa powder to the pan. Tap the pan to distribute the cocoa powder all around to coat the greased pan, and discard any excess. Set the pan aside.
In a small bowl, whisk together the flour blend, xanthan gum, baking powder, baking soda, and salt. Set the bowl of dry ingredients 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 on medium-high speed until light and fluffy (about 5 minutes).
Add the eggs, and beat on medium speed until well-combined. The mixture should be smooth and silky.
In a medium-size, heat-safe bowl, place the cocoa powder. Pour the hot water over the cocoa powder, and whisk until very smooth.
Add the melted chocolate to the medium-size bowl with cocoa powder and water, and whisk again until smooth. Set the chocolate mixture aside to cool while you finish the base of the cake batter.
To finish the base cake batter, to the butter and egg mixture in the stand mixer or large mixing bowl, add the flour mixture in 3 parts, alternating with the milk in 2 parts.
Mix until just combined after each addition, beginning and ending with the flour, until everything is just combined. Scrape down the sides of the mixer bowl as necessary. The batter should be light and smooth.
Add the chocolate mixture to the large mixing bowl with the rest of the batter. Mix until smooth and no white streaks remain, either by hand or in the mixer. The batter will be thick but soft.
Transfer the batter to the prepared bundt pan. Be sure to spread the batter into all the corners of the 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 50 minutes).
Remove from the oven and allow the cake, still in the pan, to cool upright on a wire rack for 20 minutes.
With clean hands, pull the edges away gently 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, and the pan is facing you. Tap the outside of the pan all over firmly, but not with much force, with a wooden spoon.
Allow the cake to cool for about another 30 minutes, or until the pan is no longer very warm to the touch. Remove the pan to reveal the cake.
For a cake with an even bottom, use a large knife to slice off the very bottom of the cake to level it.
Allow the cake to finish cooling completely before adding the glaze. If the cake is still warm, the glaze will slide right off.
Make the glaze
Place the chopped chocolate in a medium heat-safe bowl or 2 cup measuring cup and set it aside.
Place the cream in a small saucepan and bring it just to a simmer. Remove it from the heat as soon as it reaches a simmer. Pour it evenly over the chocolate.
Let the mixture sit, undisturbed, for 3 to 5 minutes or until almost all of the chocolate has begun to melt.
Gently stir the mixture until the chocolate is completely melted and the mixture is smooth.
Allow the mixture to cool for 15 minutes or until it's thickly pourable. The bowl with the glaze should only feel slightly warm to the touch. If it’s too thick, stir in more room temperature cream by the drop until it reaches the proper consistency.
Pour the glaze evenly over the top of the cooled bundt cake, allowing it to drip down the sides. Allow the glaze to set at room temperature or in the refrigerator for 20 minutes before slicing and serving.
Notes
Tips for melting chocolateYou can melt the baking chocolate for the batter in the microwave in 20- to 30-second bursts, whisking in between, until melted and smooth. You can also melt it in a double boiler by setting a heat-safe glass or metal bowl over a bit of gently simmering water in a small pot. Make sure the bowl on top doesn't touch the water in the pot.