Preheat the oven to 350° Fahrenheit.
Line a 9x5 or 8 1/2 x 4 1/2 loaf pan with parchment paper. Set aside. In a large mixing bowl, beat together all wet ingredients (except for the water, we'll use that in a bit!) with an electric mixer.
Add in the dry ingredients (except for the cocoa and espresso) making sure to spoon and level the flour. Beat until a smooth batter forms.
Scoop out 1 3/4 cup of batter into a medium bowl. Add in the warm water, espresso and cocoa powder. Beat together until combined.
You can also use coffee in place of the water and espresso to achieve the same rich flavor. You won't taste any coffee, it just enhances the chocolate.
There are a few methods on how to marble the loaf. What I tend to do is take 1 tablespoon of each kind of batter at a time and alternate them in the pan until each is gone.
Then I'll take a skewer and make a couple of swirls through the batter, not too many or you won't be able to see distinct sections.
Another option is to add about 1/3 of one type of batter and alternate with the other until you finish, then swirl with the skewer. Anything goes here! The beauty of a marble loaf is that each slice is different, so don't over think it too much!
When the loaf is ready, add to the oven and bake for 50-65 minutes. 60 is usually about right in my oven, but all ovens are different. If you notice the top browning too much before the center is done baking, tent the top with aluminum foil.
Remove from the oven and let cool in the pan for 10 minutes, then lift out by the parchment paper onto a cooling rack and cool for at least 2 hours.
If you want to add the chocolate glaze, add all ingredients to a small pot. Bring a second pot to a boil, then remove from heat. Place the small pot in the large pot and stir until completely melted.
Drizzle over the loaf when it has cooled. You can leave it drizzles or use a knife to spread it out. Let it harden/cool before slicing. Enjoy!