Line the bottom of three 8 inch cake pans with parchment paper (using some butter to stick the paper to the bottom). Brush/rub the sides of the pan with butter as well. Dust the sides of the cake pans with Dutch cocoa powder as well. Set aside.
Preheat the oven to 325°F (160-165°C).
Have all the ingredients measured and ready to go.
Place the hot water and cocoa powder in a jug and whisk to dissolve the cocoa powder in the water. You should have a thick and smooth paste. Stir in the buttermilk, vinegar and vanilla, and set aside.
Sift the flour, baking powder and baking soda together into a bowl. Set aside.
Place the butter in the bowl of your mixer. Using the paddle attachment, beat the butter until it’s light and fluffy, at medium speed for about 2 - 3 minutes.
Add the sugar and salt into the mix, and mix for a further 4 - 5 minutes until light and fluffy. Remember to scrape down the sides and the bottom to make sure that the sugar is well incorporated. Beat the butter and sugar mix further if needed (until light and fluffy).
Mix in the eggs, one at a time, about 20 - 30 seconds per egg. After adding 2 eggs, stop the mixer and scrape down the sides and bottom. Start the mixer again and add the final 2 eggs. Once the final egg has been whisked well into the batter, stop the mixer and remove the bowl.
The chocolate milk and flour will be mixed in by hand with a spatula, but can be done in the mixer as well (see recipe notes).
Add a generous ⅓ of the flour mix and fold it into the batter using a spatula. When the flour is mixed in half way (with some white streaks visible in the batter), add ½ of the chocolate milk mixture into the batter.
Using a spatula, fold in the chocolate mixture until it’s mixed in half way (with dark chocolate streaks now visible in the batter).
Next, add another generous ⅓ of the flour mix and fold it into the batter using the spatula.
When the flour is almost completely mixed in (some white streaks are OK at this stage), add the final bit of the chocolate milk. Fold in until almost fully mixed in (some dark chocolate streaks are also OK at this stage).
Add the final bit of flour, and fold it into the batter until it’s all FULLY incorporated. You should now have a smooth chocolate cake batter, free of lumps and white flour streaks.
Pour the batter into the prepared cake pans - about 18.7 oz (530 g) per pan. Mix and fold the batter before measuring it into each pan.
Transfer the pans into the preheated oven. Make sure the pans are not touching each other in the oven if possible (see recipe notes).
The total bake time for the cakes is between 25 - 30 minutes, but make sure to rotate the cake pan once, half way through. The cakes will be done when they are springy to the touch in the center, and the sides are just starting to pull away from the sides of the pan. A clean toothpick inserted into the middle of the cake should come out clean at this point.
Remove the cake pans from the oven and allow them to cool for about 10-15 minutes, until the cake pans can be handled.
Carefully flip the cakes out of the pan, and place them on a cooling rack (parchment side down), until they're completely cooled down. If you won’t be frosting the cake immediately, cover the cake layers with plastic wrap and store in the fridge.