Preheat the oven to 180°C/ 350°F / Gas Mark 4. Line the base and side of a 20 centimetre springform cake tin with baking paper.
Sieve the flour, cocoa, baking powder and bicarbonate of soda into a bowl and set aside.
Add the vanilla extract to the buttermilk and set aside.
Using a stand mixer fitted with the paddle attachment, beat the butter and sugars until light and fluffy, scraping down the sides from time to time.
Add the eggs, one at a time, beating well after each addition.
Add half of the flour mixture and half of the buttermilk and beat on the lowest speed until just mixed through.
Add the remaining flour mixture and remaining buttermilk and beat on the lowest speed until just combined.
Turn into the tin and gently smooth over the surface with an offset spatula.
Place in the pre-heated oven 40 to 60 minutes or until a skewer inserted into the middle comes out clean.
Remove from the oven and cool in the tin for 30 minutes before releasing the springform side of the cake tin.
Slide onto a wire rack (still in the paper) and leave to completely cool before icing.
To make the icing melt the chocolate in a heatproof bowl over a saucepan of just-simmering water, making sure the bottom of the bowl doesn't touch the water.
Using a stand mixer fitted with the paddle attachment, beat the butter until light and creamy.
Sieve in the icing sugar and cocoa, then add the salt and beat on medium speed, stopping the machine and scraping down the sides every so often, until light and fluffy.
Add the sour cream and beat until well combined.
Add the melted chocolate, taking care to beat as soon as possible as the chocolate can begin to set when it mixes with cold butter.
Beat on a medium to high speed for one minute or so, until very creamy, soft and light.
Store the cake in an airtight container in a cool, dark place for up to five days, or wrap well in plastic wrap and freeze for up to four weeks.