In a large bowl, whisk together the flour, cocoa powder, brown sugar, baking soda, baking powder and salt. Set aside.
In another bowl, whisk or mix together the oil, eggs and vanilla extract until combined. Add the buttermilk and mix again.
Pour the wet ingredients into the bowl with the dry ingredients and mix briefly on low speed until just combined. Add the cooled coffee and mix again until the batter is fully combined. The batter will be quite thin, that’s correct.
Divide the batter evenly between the two greased cake pans (I always weigh them for accuracy). Bake in a preheated conventional oven at 350°F/180°C for about 25 minutes. Check doneness with a toothpick and remove from the oven.
Let the cakes cool completely before moving on. If the cooled cakes have a slight dome, level the tops with a serrated knife. You can save the trimmings to crumble over the finished cake or for cake truffles.
Place one cake layer on a serving plate or stand and spread an even layer of chocolate buttercream on top. Add the second cake layer and frost the entire cake with the remaining buttercream.
Decorate as you like: use cake crumbs, whipped cream swirls, chocolate chips, sprinkles, M&Ms, Maltesers/Malted Milk Balls, or other toppings.