1. Preheat the oven to 180°C. Line a baking sheet with baking paper.
2. Tip the berries into a small pan with 100g of the sugar. Place over a low heat and simmer for 10-15 minutes until the fruit is soft. Sieve over a bowl to drain the syrup. Cool it, then stir in the kirsch or cherry brandy, if you're using it.
For the sponge:
1. Separate the eggs into 2 bowls. Add 100g of the sugar and 2 tsp of the vanilla to the egg yolks, and whisk until they double in volume. Set aside.
2. Using a clean whisk, whisk the egg whites into stiff peaks, then whisk in the final 25g of sugar. Fold the flour into the yolk mixture, then gradually fold in the stiff egg whites until smooth but still fluffy.
3. Using a palette knife, spread the mixture on to the baking paper so it measures about 25 x 35 cm and is level. Bake for 10-12 minutes until golden. Cool for 5 minutes, then brush the sponge with the syrup. Cover with clingfilm, and set aside.
For the white chocolate cream:
1. Melt the pieces of white chocolate in a heatproof bowl set over a pan of barely simmering water. In a small pan, gently heat the cream but don't let it boil.
2. Pour the cream over the melted chocolate, whisking gently until smooth. Stir in the vanilla and mascarpone until glossy, then add the lime zest. Leave to chill for 2 hours.
3. Using an electric hand whisk, beat the chilled white chocolate cream to peaks - but don't overdo it as the mixture will split.
4. Use a palette knife to spread a thin layer of the cream over the soaked sponge, then cover with the berries.
5. Use the baking paper to roll up the Yule log tightly, starting with one of the long sides. Slice off a piece of the log to make a branch, remove the paper and place the log on a serving platter with the branch at an angle to the log.
6. Cover the log with the rest of the white chocolate cream. Use a fork to create the bark effect. Decorate with redcurrants and dust with icing sugar. Chill until ready to serve.