Preheat the oven to 180°/160°C fan/350°F/gas mark 4. Grease and line two 20 cm/8 in round cake tins.
Whisk together the oil, Guinness, yogurt, vinegar, vanilla and sugars in a large bowl until smooth.
Sift together the flour, cocoa powder, baking powder, bicarbonate of soda, and salt.
Add the dry ingredients to the wet and stir with a balloon whisk until no dry lumps remain.
Divide the batter evenly between the tins. Bake for about 30 minutes until a skewer inserted into the centre comes out clean.
Leave the cakes to cool in the tins for 10 minutes then carefully turn them out onto a wire rack and leave to cool completely.
To make the ganache, place the chopped chocolate in a heatproof bowl. Heat the cream and Guinness together in a pan until almost simmering but don't let it come to the boil.
Pour the cream over the chocolate, let stand for 5 minutes then stir gently until smooth. Stir in the melted butter. Set aside until it becomes thick and spreadable. If you accidentally leave it too long and it becomes too thick, you can gently re-warm it over a pan of hot water (don't let the base of the bowl touch the water).
Meanwhile make the buttercream. Whisk the butter, icing sugar and whiskey together with an electric mixer until light and fluffy.
To assemble the cake, trim the tops of the cakes to level them if necessary. Place one cake layer on a cake stand or serving platter and spread over the buttercream. Place the other cake layer on top and spread over the ganache.