Separate the egg yolks and egg whites and set aside.

Whisk the egg yolks and vanilla together by hand until combined. Add the flour and baking powder, and whisk until smooth.

Add the milk to the egg yolk mixture and whisk until combined. Add the oil and whisk until no lumps remain. Set aside.

Preheat the oven to 285°F (140°C), top and bottom heat, no fan.

Beat the egg whites on high speed until foamy. Gradually add the sugar and salt in two additions, beating until soft peaks form and the mixture is glossy.

Gently fold the egg white mixture into the egg yolk batter until fully combined.

In a separate bowl, mix a portion of the batter with the cocoa powder dissolved in hot water. Stir until well combined.

Alternately spoon the vanilla and cocoa batters into a prepared loaf pan. Use a skewer or toothpick to swirl and create a marble pattern on top.

Bake the cake for 35 minutes. Check for doneness by inserting a toothpick into the center; if it comes out clean, the cake is done.