120 g unsalted butter, softened, plus extra to grease
250 ml cola
250 g dark brown sugar
120 g caster sugar
80 g cocoa powder
4 large free-range eggs
200 ml buttermilk
400 g self-raising flour, (you might start with 320 g first; as you mix if the consistency is watery add more flour)
1 tsp bicarbonate of soda
For the chocolate frosting
400 g Lindlt 70% chocolate, roughly chopped
100 ml cola
100 ml water
250 g unsalted butter, diced
50 g dark brown sugar
Cocoa powder to dust (optional)
Pre heat the oven to 160°C. For the sponges, butter 24 cm tube tin. In a jug, stir the 250 ml cola and 250 g dark brown sugar until the sugar has more or less dissolved. Set aside.
Using an electric mixer, beat the 120 g butter and caster sugar for 5-7 minutes until pale and light in texture. Add in the cocoa powder and beat again to form a brown paste.
In a separate jug, whisk the eggs with the sour cream until smooth, then slowly pour into the cocoa paste, beating constantly. Sift the flour and bicarbonate of soda over the mixture and beat in. Pour the cola-dark brown sugar mixture into the bowl and beat to a smooth, fairly loose batter.
Pour into the prepared cake tins and bake for 40- 45 minutes until a skewer inserted into the centre comes out clean. Cool the sponges in the tin for 15 minutes, then invert onto a wire rack to cool.
For the frosting, put the chocolate into a large heatproof bowl. Heat the cola, water, butter and dark brown sugar in a pan until the butter melts completely and the mixture starts to boil. Remove from the heat and pour over the chopped chocolate. Leave the mixture to rest for a minute so the intense heat of the liquid melts the chocolate, then bring together with a balloon whisk until smooth and glossy.
Cover the frosting with a sheet of cling film directly touching the surface, allow to cool completely, stirring occasionally, until stiffened to a spreadable consistency – it should take around an hour.
Once the sponges have cooled completely, and the frosting has thickened, place the sponge onto a cake stand line with foil preferably a turning cake server. Using a palette knife, spread the frosting, cover the entire cake, swirling, creating designing snow balls and smoothing as you go. Decorate with mixed berries and candles if you are making it a birthday cake. Serve, or leave somewhere cool or in the fridge until ready to serve (see Make Ahead).
Make the sponges up to 24 hours ahead and keep in an airtight container in a cool place, or freeze wrapped in cling film for up to 1 month. The finished cake will keep chilled, covered with cling film, for up to 4 days. Bring to room temperature before serving.