To make the marble cake layers:

1. In a bowl, whisk together hot water and cocoa powder until the cocoa powder has dissolved. Set aside to come to room temperature.
2. In a mixer with a beater attachment, mix butter with sugar on medium speed until pale and creamy, about three minutes. Scrape down to the bottom of the bowl once during mixing to make sure no butter or sugar is stuck at the bottom.
3. Add eggs one at a time, mixing on the lowest speed after each egg until incorporated, about 30 seconds.
4. Add vanilla and mix on lowest speed for about 30 seconds to combine.
5. In a bowl sift and whisk together 1 cup flour with baking soda, baking powder and salt. Add to the mixing bowl and mix on the lowest speed to incorporate.
6. Pour or scoop half of the batter into a separate bowl. You can pour it straight into a measuring jug (it will be be 1 1/3 cups batter) or into a bowl on a scale (it will weigh 325g). Set this aside to make the chocolate batter later.
7. Add the buttermilk and oil to the mixing bowl and mix on the lowest speed to combine. Add 1/2 cup flour and mix on the lowest speed until you can't see flour in the batter, about 30 seconds.
8. Grease three cake pans and divide the vanilla batter between them, using an ice cream scoop or spooning it into blobs spread out around each pan.
9. Pour the set-aside batter back into the mixing bowl and add the cocoa powder mixture. Mix on the lowest speed to combine. Add 1/2 cup flour and mix until batter is smooth, about 30 seconds.
10. Using a measuring cup or ice cream scoop, divide the chocolate batter between the pans by adding it in blobs in between the vanilla batter. Use the handle of your spatula or a toothpick to swirl the batter, going all the way down to the bottom of each pan. Tap each pan on the counter to knock any air bubbles out of the batter.
11. Bake at 175ºC or 350ºF for 26 minutes or until a toothpick inserted into the middle of the cake comes out clean.
12. Let cakes cool in their pans for 5 minutes. Use a spatula to loosen cake from the edges of the cake pan and then turn onto a wire cooling rack. Leave to cool completely.

To make the frosting:

1. Mix butter and powdered sugar on the lowest speed until smooth. Add vanilla, salt and milk and mix to combine.
2. Heat the chocolate chips in the microwave at 80% power for 30 seconds at a time, stirring after each time, until smooth. Set aside to cool.

To put it all together:

1. Spread a dot of buttercream onto the middle of a cake board and press your first layer of cake onto it. Spread vanilla buttercream onto the first layer of cake and place the next layer on top. Repeat with another layer of filling and the final cake layer.
2. Set aside 1/2 cup of vanilla buttercream. Add the cooled melted chocolate to the rest of the buttercream, stirring until there are no streaks of melted chocolate in the buttercream.
3. Frost the cake with chocolate buttercream, applying a thin layer first (a crumb coat) and letting it set at room temperature for 30 minutes or in the fridge for 15 minutes before applying a final layer.
4. As soon as you've finished frosting the cake, use the remaining vanilla buttercream to spread or pipe a thin layer around the bottom of the sides of the cake. Using gentle pressure, rest an offset spatula or the back of a spoon on the cake board and drag it diagonally upwards to create texture in the vanilla buttercream, repeating the motion all around the cake.