Preheat the oven to 350°F (180°C) and position a rack in the center. Grease an 8 × 4-inch loaf pan, then line the bottom and sides with parchment paper.
Add the flour, sugar, and baking powder to the bowl of a stand mixer fitted with the paddle attachment. Briefly whisk to evenly distribute the dry ingredients.
Pour in the heavy cream, oil, buttermilk, eggs, salt, and vanilla extract. Mix on low speed just until combined, then increase to medium-high and beat for about 2 minutes. Scrape down the bowl, reduce the speed, and mix for an additional 30 seconds.
Transfer 1 cup of the vanilla batter to a separate bowl. Whisk in the cocoa powder and water until smooth to create the chocolate batter.
Spoon about one-third of the vanilla batter into the prepared pan, then add one-third of the chocolate batter in a strip down the center. Repeat the layering until all the batter is used.
Firmly tap the pan on the counter several times to release any trapped air bubbles.
Bake for 60–65 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Let the cake cool in the pan on a wire rack for 20 minutes, then remove it and let it cool completely, about 2 to 2½ hours.
To make the fudge frosting, place the chocolate chips and oil in a microwave-safe bowl. Heat in 15-second intervals, stirring between each, until melted and smooth.
In a stand mixer, beat the softened butter on medium speed until smooth and glossy. Scrape the bowl, then add the cocoa powder, confectioners’ sugar, salt, vanilla, 2 tablespoons of the heavy cream, and the melted chocolate.
Mix on low until combined, then beat on medium speed for about 1 minute. If needed, add the remaining tablespoon of cream to adjust the consistency. Scrape the bowl and mix briefly on low until smooth.
Once the cake has fully cooled, use a serrated knife to slice it horizontally. Spread about half of the frosting over the bottom layer, place the top layer back on, and frost the top with the remaining frosting.