Preheat the oven to 170 C. Grease a 9x5 loaf tin (see notes) and set aside.
Sift the flour, baking powder and salt together in a bowl. Set aside.
In a mixing bowl, beat the butter and sugar with a hand mixer until pale and fluffy. Add the eggs and vanilla and beat again to combine. If the eggs are cold, the batter may curdle a little bit so if this happens, don't worry, it will smoothen out in the next step. Preferably, use room temperature eggs.
Fold in the flour mixture and stir in the milk. The batter will be a little thick which makes it easier to do the marbling, so don't add more than 2 tbsps of milk.
Take out half the batter into a small bowl. Into the other half, sift in the cocoa powder and stir well to combine. You can stir in an additional 1 to 2 tbsps of sugar into the chocolate batter.
Now drop alternating spoonfuls of both batters (see photos above) into the prepared cake tin. Using a toothpick or skewer, gently drag the vanilla and chocolate into swirly patterns on the top. Poke the toothpick further into the batter to get some swirls into the rest of the cake as well.
Bake for 30 minutes, or until a toothpick poked in the center comes out clean. Start checking for doneness at 25 minutes.
Allow the cake to cool in the tin for 20 minutes, then carefully remove it and place on a wire rack to cool completely. Slice and enjoy! Store in an airtight tin at room temperature for 3 to 4 days, and in the fridge for a week. Happy baking!