Grease an 8 1/2 inch x 4 1/2 inch loaf pan, sprinkle evenly but lightly with cocoa powder, sugar and ground cinnamon and set the pan aside.
In a small saucepan, place the chopped chocolate and milk over medium heat, stirring occasionally, until the chocolate is melted and the mixture is smooth. Set the mixture aside to cool briefly. In a medium-sized bowl, place the flour, xanthan gum, salt, cocoa powder and cinnamon, whisk to combine well and set the bowl aside.
In the bowl of a stand mixer fitted with the paddle attachment (or a large bowl with a hand mixer), beat the butter and sugar until the mixture falls from the beater in a ribbon when the paddle is lifted (about 3 minutes). Add the eggs, one at a time, and the vanilla, and beat again until well-combined. Next, add the chocolate and milk mixture, and beat again until combined. Finally, with the mixer on low, add the flour and cocoa powder mixture slowly. Beat briefly until well combined. The mixture should be smooth and thickly pourable.
Pour the mixture into the prepared pan, and smooth the top with a wet spatula. Sprinkle the top lightly with sugar, cinnamon and cocoa powder. Place in the center of the preheated oven and bake until a toothpick inserted in the center comes out with no more than a few moist crumbs attached (about 1 hour).