Thoroughly grease a 10 cup bundt tin, making sure that you get into all the nooks and crannies. Dust the tin with cocoa powder and shake out the excess. Heat the oven to 160°C/320°F/gas mark 3.
Sift together the flour, baking powder, bicarbonate of soda and salt and set aside. Melt the chocolate, either over a pan of gently simmering water or in short bursts in the microwave. Sift in the cocoa powder then add the vanilla extract, sour cream and boiling water and whisk until smooth. Set aside.
Place the butter and sugar in a large bowl and whisk with an electric mixer until very pale and fluffy. Whisk in the eggs one at a time, adding a spoonful of the flour with each one. Whisk in the chocolate mixture then finally fold in the rest of the flour mixture.