Preheat your oven to 325 degrees F. Spray a 9x5-inch loaf pan with nonstick spray. Line the pan with parchment paper, allowing the parchment to come up the sides of the pan. Spray the parchment. Set aside.
In a small saucepan, combine ⅓ cup sugar, the water, cocoa powder, and chocolate. Stir over medium heat until the chocolate is melted and the mixture is smooth. Remove from heat and let cool while you make the cake batter.
In a medium size mixing bowl, whisk together the flour, baking powder, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and remaining sugar on medium speed until light in color and fluffy in texture, about 2 to 3 minutes.
Add the eggs, one at a time, making sure to scrape down the sides and bottom of the bowl in between additions. Stir in the vanilla extract.
Continue to beat on medium-high speed for about 2 minutes, until the mixture gets smooth in texture and slightly bigger in volume.
With the mixer on low speed, add 1/3 of the flour mixture, followed by 1/2 of the buttermilk, another 1/3 of the flour, the second half of the buttermilk, and then the remaining flour mixture. Stir until just combined.
Scrape down the sides and bottom of the bowl and mix again for another 20 to 30 seconds on low.
Pour about 10 ounces of batter in a separate mixing bowl. Gently fold the chocolate mixture into the new bowl of cake batter.
Alternately add the vanilla and chocolate cake batters to the prepared pan. I like to use a small measuring cup to scoop the cake batter into the pan so I can easily layer the different flavors. Use a toothpick or knife to swirl the batter, but don’t mix it too much - you want the flavors to be distinct and separate.
Bake for 45 to 50 minutes or until a toothpick inserted into the center comes out with a few moist crumbs on it.
Remove from oven and let cool in the pan for about 15 minutes before inverting onto a wire cooling rack to cool completely.