Preheat oven to 350F/175C. Butter a 9x5-inch loaf pan.

In a large bowl sift together flour, baking powder, baking soda and salt. Add sugar and mix. Set aside.

In a medium bowl, whisk the eggs with the buttermilk, orange juice and zest, melted butter, oil and vanilla. Pour the wet ingredients into the dry ingredients and stir with a wooden spoon or a rubber spatula just until combined. Do not over mix - the less you mix, the lighter the cake will be.

Divide batter into two and fold the melted chocolate to one of the two mixtures.

Dollop the orange and chocolate mixtures alternately into the pan (the post includes a photo to demonstrate it) - first, next to each other (like chess board), then, in layers on top of each other. When done, insert a knife to the mixture all the way down, and start swirling the batter gently, just don’t mix to the point where all the batter turns chocolaty.

Bake for about 45-55 minutes until a toothpick inserted into the center of the cake comes out clean. Let the cake cool for 10 minutes on a wire rack, then turn it out from the pan and let it cool completely.