Preheat the oven to 350F. Butter and flour a 12-inch bundt cake, tapping out excess flour (I prefer using Bakers Joy spray)

Make the chocolate swirl: In a small pan over high heat, simmer water, sugar and cocoa, mixing until smooth and fully combined. Remove from heat, add the chocolate chunks/chips and whisk until incorporated. Set aside to cool.

In a medium bowl, whisk flour, baking powder, baking soda and salt.

In the bowl of a stand mixer fitted with the paddle attachment, cream butter and sugar on medium-speed, until light and fluffy.Add eggs, one at a time, and vanilla extract, scraping down sides of bowl after each addition.

Reduce speed to low. Add the dry ingredients in 2 additions, alternating with the sour cream. Beat just to combine.

Transfer about 2 1/2 cups of the batter to a separate bowl. Add the cooled chocolate mixture and whisk until fully incorporated.

Pour half of the remaining vanilla batter onto the prepared bundt pan, followed by the chocolate batter, and top with the remaining vanilla batter. Using a knife, make swirls in an 8-figure motion, going around the cake about 2-3 times.

Bake for 70 minutes, or until a toothpick inserted in the center comes out clean.

Let cool for 30 minutes in the pan. Run a small angled spatula or knife around the cake to loosen it before flipping onto cake plate or stand.

In a large bowl, sift the powdered sugar and cocoa powder. Add the canola oil and one tablespoon of hot water at a time (I used about 5). Use a spoon to mix the ingredients, and work the spoon against the bowl to get out any of the clumps. I prefer thick icing over runny, so feel free to add more boiling water until you get your desired consistency. To get even coverage throughout the cake, transfer the icing to a piping bag, no tip necessary, and go around the cake as needed.