1. Preheat the oven to gas 4, 180°C, fan 160°C. Grease and line 2 x 20cm cake tins with baking paper.
Melt the butter and chocolate in a heatproof bowl over a pan of barely simmering water. Remove from the heat

2. Add the sugar to the chocolate and whisk with an electric whisk for 2 mins.
Add the eggs and crème fraîche and whisk for 2 mins more until smooth and combined.
Fold in the flour, baking and cocoa powders; whisk for 1 min until just combined.

3. Divide the batter between the tins and bake for 30-35 mins until a skewer inserted in the centre comes out clean.
Leave to cool for 10 mins in the tins, then transfer to a wire rack to cool completely.

4. Meanwhile, whisk together the butter, icing sugar and cocoa powder in a large bowl
with an electric whisk for 2-3 mins until light and fluffy.
Add the cooled melted chocolate and milk, then whisk for 2-3 mins more until smooth.

5. Spoon the icing into a piping bag fitted with a large nozzle
then start piping from the outside edge of one of the upturned sponges in a spiral to the centre.
Smooth with a palette knife, then pipe a second layer around the edge to make a border.

6.Store the cake in an airtight container in a cool, dark place for up to 5 days