Preheat your oven to 350, grease a nonstick loaf tin (9*5)
Combine the first three ingredients in a bowl, set aside
Use a mixing bowl to whip the oil, sugar and eggs together on high speed for about two minutes. The mixture will be a creamy pillowy yellow
Add the vanilla, beat for 30 seconds to combine.
Alternately add the flour and milk to the bowl in two batches and mix until just combined.
Put a cup of the batter into a small bowl, add the 3 tbsp cocoa and 1 tbsp milk and mix until just combined.
The batter for this cake is thin, so carefully add it to the loaf pan, alternating between dollops of vanilla and chocolate batter
Use a long skewer or a knife to swirl the batter together
Bake for an hour or until a toothpick inserted in the middle comes out clean.