Preheat the oven to 160 degrees C (325F) and grease your bundt pan. (I've recently found Wilton Bake Easy and I now couldn't live without it!! It's much easier than greasing with butter, especially when you have odd shaped pans like a bundt pan.) Lightly dust the greased pan with cocoa powder or flour. (I used cocoa powder)
Break the chocolate up and place in a heatproof bowl. Place in the microwave in 15-20 second increments, stirring in between each one until melted. If you don't like melting chocolate in the microwave, place the heatproof bowl over a simmering saucepan of water and stir until melted.
Sift the flour, cocoa, baking powder, bicarb and salt together in a medium sized bowl and leave to one side.
Place the butter and sugar into a large bowl or the bowl of your stand mixer. Beat until light and fluffy, about 2 minutes on med-high speed.
Add in the eggs and vanilla extract.
Mix until well incorporated, smooth and fluffy, about 2 minutes. Add in the dry ingredients and mix until well combined.
Add in the sour cream and melted chocolate and mix until well combined and smooth.
Place the batter into your bundt pan, smooth the batter and try and push the edges up about 1/2 inch higher than the middle of the pan.
Place in the oven for 50-60 minutes until risen and an inserted skewer comes out clean.
Leave to cool completely in the bundt pan.
Set up a wire rack with paper towels underneath to catch the frosting. Flip the pan over the wire rack and tap and shake until the cake comes out, if you've greased the pan well, the cake should come out easily!
To make the frosting, place the chocolate into a heatproof bowl and melt, either in the microwave or over a simmering saucepan of water. Once melted removed from the heat and place to one side.
Sift the icing sugar into a medium sized bowl and add in the vanilla extract. Pour in the melted chocolate and then beat the frosting together until combined, it probably won't come together at this stage, but that's fine. Add 2 tbsp of water to the mixture and mix until combined. After this, add in water 1 tablespoon at a time until you have a thick, but pourable frosting.
I placed my frosting into a jug and then poured it all over the cake, but you could spoon it over.
Leave the frosting to set for an hour before transferring the cake to a plate. I highly recommend you use a cake lifter to move the cake as it's quite heavy and prone to breaking if you just try and pick it up.
Will keep in an airtight container in the fridge for up to 5 days, at room temperature for 3 days. Serve at room temperature.