Pre-heat the oven to 160 C (325 F). Cover a cake pan with butter. In a medium sized bowl, mix together with a whisk the baking powder, salt and flour.
In this part you need an electric mixer. Use the mixer to beat the butter and the sugar in a large bowl until it is starting to look fluffy. Do this for about 3 minutes. Reduce the electric mixer to low speed and mix in the eggs. As you put in the eggs one at a time, ass the vanilla extract.
You can add the flour mixture and the milk mixing until it's added into the mixture.
In another medium sized bowl, combine hot cocoa mix with boiling water. Add one half of a cup of batter and then mix together. One third of vanilla batter will be spooned into the prepared pan with one third of the chocolate batter. Repeat this process twice. With a butter knife, use swirling motions a couple of times. (Don't do it too much as It can appear muddy)
Bake your cake until you can see that when you put in a wooden pick, it comes out clean. Bake for 70-75 minutes. Let the cake cool in the pan for about 5 minutes and then put on a wire rack to continue to cool until it is edible.