Preheat the oven to 325 degrees and grease and flour a 9 x 5 standard loaf pan. Make the chocolate base: Whisk the cocoa powder, espresso powder, sugar and water together in a medium bowl until smooth until smooth. Set aside.
Make the cake batter: Sift the flour and baking powder together (a fine mesh strainer works great for this job) onto a clean piece of parchment paper. Set aside.
Combine the sugar, canola oil, vanilla and salt together in the work bowl of a stand mixer. Mix on medium speed until just combined. Add eggs and egg yolk, one at a time, making sure that each is fully incorporated before adding the next. Continue to mix on medium high until thick in texture and pale in color, about 3-5 minutes.
Reduce the mixer speed to low. Add one third of the flour mixture followed by one half of the buttermilk. Add the second third of the flour mixture followed by the remainder of the buttermilk. Finally, add the remaining third of the flour mixture. Mix until the cake batter just comes together, being careful not to overmix.
Transfer about half of the batter to the bowl with the cocoa powder mixture and fold together until the batter is consistently brown in color.
Add one third of the plain batter to the prepared pan followed by one third of the chocolate batter. Repeat twice more, alternating plain and chocolate batters.
Bake until a tester inserted in the center of the cake comes out clean, about 1 hour and 20 minutes to 1 hour and 40 minutes. Allow the cake to cool in the pan for 20-30 minutes and then run a knife around the sides of the cake to make sure it is not stuck to the sides. Carefully transfer the cake to a wire rack to cool completely before serving.
Notes
This cake keeps well tightly wrapped in plastic at room temperature for about 3 days. For longer storage, wrap the completely cooled cake in plastic and then in a layer of foil and freeze for up to 3 months.