This cake is crazy-easy and stays fresh in the fridge for three days.
Though it has a high sugar content, the bitter Guinness and sour buttermilk provide balance and, together, they are why the cake stays fresh for so long. It’s a great birthday cake – the one I make most for people – that you can make well in advance, as both the sponge and the cream freeze very well. Even my son has been fed it, as the alcohol is cooked off. This is a very wet batter, so don’t worry: that’s the way it’s supposed to be.
Serves 10–12
For the sponge
For the cream
Place a saucepan under an extraction fan, pour in the Guinness and set over a high heat. Reduce to half the quantity (250g). Set aside to cool.
Melt the butter in a pan, then leave it to cool.
Butter 2 swiss roll tins, each about 30 × 20cm, and line the bases with baking parchment. Preheat the oven to 160°C fan. Place the flour, sugar, cocoa, bicarb, baking powder and salt in a bowl, whisk to mix, then make a well in the centre.
Add 125g of the Guinness reduction with the cooled melted butter, egg and buttermilk and whisk the batter until no lumps are left.
Divide the mixture between the 2 prepared tins, then bake for 10 minutes. Insert a metal skewer into the centre: when it emerges, it should be clean; also the sponge should bounce back to the touch. It should be just cooked, rather than overcooked.
The mix is wet and you really need to use a metal skewer to check it’s done in the centre. Leave in the tins to cool, then put a rack over the tins and turn them out. Leave until cold, then chill. The sponge needs to be chilled when you’re cutting it to assemble the cake.
Halve both chilled cakes widthways, then trim the sponges so you have 4 matching flat layers that will sit neatly on top of each other. Whip together the cream, icing sugar, buttermilk, mascarpone and vanilla seeds to soft peaks (see below).
Use this to sandwich the cake layers together. Serve in slices, so you see the layers.
Tricks of the trade
Adding mascarpone to double cream is an amazing tip given to me by a fantastic pastry chef, Rey (Hortillosa) Encarnacion from the Conrad Hotel in Dublin. When you add mascarpone to whipped cream, it never loses its air and is less likely to split