Preheat the oven to 350°F/177°C.
In a large mixing bowl, combine all the dry ingredients: flour, cocoa powder, salt, baking soda, and baking powder. Set aside.
Cream butter and sugar in another large bowl using an electric mixer (a stand mixer works just as well).
Add the eggs one at a time, mixing well after each addition.
Next, add sour cream, vanilla, and buttermilk, and continue mixing until all the ingredients have been thoroughly combined. Add the dry ingredients to the bowl with the wet ingredients. Mix well.
Pour in the hot water (depending on consistency) and mix until it's fully incorporated into the batter.
Divide the cake batter evenly between three 8-inch cake pans. For accurate measurements, I weigh each pan and adjust accordingly.
Place the cake layers in the oven and bake for 20-22 minutes or until a toothpick inserted into the center comes out clean. Remove and let them cool completely. I usually make these a day or more in advance, wrap them and freeze them. When ready to make the cake, remove and assemble them.