Preheat oven to 350 F. Spray a 9x5 inch loaf pan and set aside.
In a large bowl whisk together all dry ingredients until combined. In another bowl, whisk together egg, buttermilk, melted butter, vanilla and hot water. Add the liquid ingredients to the dry ingredients and mix just until combined. Reserve 1 cup of the batter.
Melt together chocolate chips and 1 tbsp of water. Stir melted mixture into the reserved 1 cup of batter until uniform in color. Pour half of the vanilla batter into the loaf pan. Pour the chocolate batter on top and finally, pour the remaining vanilla batter. Use a knife and drag it across the batter 2 times to create the marbled effect. Sprinkle sugar on top.
Bake in the preheated oven for 50-60 minutes or until a toothpick inserted in the center of the cake comes out clean. Cool in a wire rack for ten minutes. Then remove from the pan and cool on the wire rack completely.