Chocolate Guinness sponges:

1 Preheat the oven to 195 degrees Celsius or 175 degrees if using a fan oven. Grease and line the base of 4 x 20cm sandwich tins with a disc of parchment paper.

2 Melt the butter in a medium saucepan and set aside to cool. Add the Guinness and whisk well, then sift in the cocoa powder and whisk until the cocoa has dissolved. Set aside to cool.

3 In a bowl, whisk the eggs and sour cream or yoghurt (if using) together until smooth. Add the cooled Guinness mix, followed by the flour, sugar and baking powder. Whisk until just combined, then divide the cake batter between the four tins and smooth the tops using a palette knife or back of a spoon.

4 Bake the sponges for 30-35 minutes, until a skewer inserted into the centre comes out clean. Remove from the oven and leave to cool in the tins for 10 minutes before turning out onto a wire rack and leaving to cool completely.

Champagne buttercream:

1 Put the champagne or sparkling wine into a small saucepan and place over a medium heat.

2 Simmer for five or so minutes, or until it has reduced by two-thirds; you should have about 65-75 ml left, or about 4-5 tbsp. Pour into a jug and leave to cool completely.

3 Make the buttercream by beating the butter until pale and soft. Add half of the icing sugar, beat until well combined, then add the remaining half. Beat well for about three to five minutes, until light and fluffy. Add the vanilla and slowly beat in the reduced champagne, then continue to beat until well combined.

To assemble:

1 Layer the sponges with the buttercream, then finish by icing the top and sides of the cake to serve.