Preheat oven to 300° F
Butter an 8" cake pan and line it with parchment paper
Place the chocolate and milk in a heatproof bowl and set over a pan of barely simmering water, ensuring that the bottom of the bowl doesn't touch the water
Once melted, let cool slightly
Sift the flour, salt, and cocoa powder together before setting aside
Beat the butter until soft
Add the sugar and beat until creamy and light
Add the eggs one at a time, beating thoroughly after each addition
Using a rubber spatula, fold in the melted chocolate and then the flour and cocoa mixture until just combined, without overworking the batter
Pour into the prepared pan, smooth the top and bake for 35-40 minutes, until a skewer inserted in the center comes out clean
Let cool completely in the pan, before turning it out
Chocolate Buttermilk Frosting
In the meantime, make the icing by whisking together the powdered sugar, cocoa, and buttermilk. Really go at it for at least a minute.
The icing should end up smooth and creamy looking, adjust with a touch of powdered sugar or a few extra drops of buttermilk if you want to tweak the consistency at all.
When the cake is completely cool, run the icing around the top and sides with an offset spatula and decorate