To make homemade buttermilk, mix whole milk and vinegar in a bowl. Set it aside for about 5 - 10 minutes until it curdles.
10.5 oz Whole milk, 0.7 oz White Vinegar
Coat a layer of soft butter inside the pan and attach a cake liner (or parchment paper) to the bottom.
Some Butter (soft), oil spray
Mix all the dry ingredients with a whisk.
14.1 oz Granulated sugar, 7.6 oz All-purpose flour, 3.5 oz Cocoa powder, 2 teaspoon Baking soda, 1 teaspoon Baking powder, ½ teaspoon Salt
Mix all the wet ingredients in a large bowl with a whisk. The buttermilk should be ready by then!
3.5 oz Egg, 4.4 oz Oil, 1 teaspoon Vanilla extract
Add the dry ingredients to the wet ingredients and mix with a rubber spatula until smooth.
Heat water and butter in a small saucepan just until it starts simmering. Add ⅓ - ½ of it to the chocolate mixture and mix. Add the rest and mix. Scrape off the sides and bottom of the pan to ensure it's evenly mixed.
2 oz Unsalted butter, 10.5 oz Hot water
Pour the cake batter into the prepared pans. (540 g | 19 oz each for 3 layers)
Bake at 350ºF (176ºC)for about 30 minutes with 3 8" pans until the center bounces back gently or a toothpick comes off clean. Adjust the baking time and temperature depending on how thick the cake is, the size of the pan, the oven, etc. Cool them on a wire rack before taking them out of the pans or flip the pan VERY gently since the cake is fragile while it's hot.