Preheat oven to 175°C (350°F). Grease + line three 9-inch round cake tins.
Sift together flour, cocoa, baking soda, baking powder, and salt.
In another bowl, beat sugar, oil, and eggs until light (2–3 mins).
Mix in sour cream + vanilla, then add dry ingredients slowly.
Pour in hot coffee + milk gradually on low speed (batter will be thin, that's what makes it moist).
Divide into tins, bake 30–35 mins (check at 25–28 mins) until a skewer comes out with moist crumbs.
Cool 10 mins in tins, then turn out to cool completely.
Put chocolate in a heatproof bowl.
Heat cream and honey until just starting to simmer.
Pour over chocolate, rest 1 min, then stir from the center out until glossy.
Stir in butter until melted.
Sift in cocoa + icing sugar, whisk until smooth. (Don't overmix, keep it silky, not airy.)
Cool at room temp for 30 mins, then spread generously between and over cake layers.
