In a large bowl, sift together 1.25 cups of all-purpose flour (keep 0.25 cups aside for later), baking powder and baking soda. Keep aside
In another bowl, add in your butter (at room temperature) and sugar, and mix it together using your whisk, or preferably a hand beater, until light and fluffy.
Add in your eggs, one at a time, and mix well. Add in your vanilla at this stage too, and mix well.
Alternatively, add in your buttermilk and flour mixture, starting and ending with the flour mixture.
After the wet and dry ingredients are combined well, carefully divide this mixture into two parts, in 2 separate bowls.
In one of the bowls, add in the 0.25 cups of flour that we kept aside in the beginning, and mix well.
In the 2nd bowl, add in 1/4 cup of cocoa powder, and mix well.
Preheat your oven to 180C.
Take a loaf tin, and grease it well with butter.
Using a cradle, or any large spoon, alternately add both the mixtures, vanilla and chocolate, to the loaf tin, and keep swirling it with a toothpick to have the marble effect.
Bake for 30-35 minutes, or until a toothpick inserted comes out clean.
Let it cool completely in the pan, before slicing