Preheat the oven to 350 ºF. Butter and flour a 8½" x 4½" loaf pan OR a 9"x5" loaf pan, then line the bottom with a rectangle of parchment paper. Set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. If the flour seems lumpy, you can sift it through a metal sieve to get out all the lumps!
In a medium bowl, using a whisk, combine the cocoa powder and the boiling water until it is smooth. The consistency will be like thick melted chocolate. Set aside to cool down slightly before using.
In a large bowl, using an electric hand mixer or in the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugars for at least 5 minutes until light and fluffy. Be sure to scrape down the sides of the bowl often to make sure the mixture is evenly whipped.
Add the eggs, one at a time, mixing in between each addition and scraping down the bowl as needed.
Add the vanilla, sour cream, and warm melted cocoa mixture. Mix it in until all the ingredients are evenly mixed.
Add the sifted flour mixture and stir it in on low. You may want to finish mixing with a spatula to make sure the ingredients are all combined.
Transfer the chocolate loaf cake batter to the prepared loaf pan. Bake the cake on the middle rack until baked through. You'll know it's done because it will have cracked a little in the middle, it will pull away from the edges of the pan, a cake tester will come out clean, and the internal temperature will register around 100 ºC (212 ºF). It takes about 65 to 75 minutes to bake this cake in my experience.
Let the cool cake in the pan on a wire rack, then unmould it to cool completely before slicing (at least 2 to 3 hours).
Serve plain as a snack, or top with vanilla bean ice cream and fresh berries.