Preheat the oven to 350 degrees F, and line an approximately 11-inch loaf pan parchment paper. (Use a large piece that will cover all 4 sides and cut slits in each corner so that it will lay flat.) Set aside.
In a large mixing bowl, combine the gluten-free flour, cocoa powder, and baking soda. Make a well in the center and set aside.
Add the avocado chunks to a medium-sized mixing bowl, use the back of a fork to mash them until they are as smooth as possible. Add the oil, sugar, salt, milk, yogurt, and vanilla. Whisk this until everything is evenly incorporated. Then whisk in the egg until it's completely blended in.
Pour the wet ingredients into the well in the dry ingredients and mix only until combined — until you no longer see any dry spots.
Fold in the hot coffee. Once it's blended in, fold in the chocolate chips or chunks.
Pour the batter into the prepared pan, and bake in the preheated 350 degree F oven until the cake becomes solid (it should no longer jiggle if you move the pan around), and is cracked on top, about 1 hour.
Let the cake rest in the pan for at least 20 minutes or so. It will sink in the middle — it's just what this cake does.
Use the parchment paper surrounding the cake to carefully lift it out of the pan. Wait about an hour before slicing the cake.