Grease and line a 9x5inch (22x13cm) loaf pan or grease and flour a 20cm bundt pan. Preheat oven to 350°F/180°C.
Combine together the flour, baking powder, and salt and set aside. Dissolve baking soda in buttermilk and set aside.
In the bowl, beat the butter and sugar until light and fluffy, about 5 minutes. Add and beat eggs, one at a time, until well combined. Scrape down the sides of the bowl from time to time. Add vanilla extract. Add half of the flour mixture then the buttermilk and then the other half of flour mixture. Mix well. Set aside 1/3 of the batter.
In a smaller bowl, mix cocoa and 1/4 cup plus 2 tablespoons boiling water until smooth. Add the reserved 1/3 batter to the cocoa mixture and mix well.
Spoon vanilla batter into the prepared pan to form the first layer then alternate with a thin layer of chocolate batter. Lightly tap the pan on the counter surface 2-3 times every after addition of a layer. Repeat this process until all batter are used up. This will create a cleaner marble effect than swirling a knife through the batter.
Bake for 40-50 minutes or until the toothpick inserted in center comes out clean.
Transfer pan to a rack to cool 10 minutes. Turn out the cake from pan and cool completely on the rack.
Cake can be kept in an airtight container at room temperature up to 3 days.