Preheat the oven to 160°C/140°fan/Gas 3. Line a 28 x 18cm (11 x 7in) Swiss roll tin with baking paper large enough to come just higher than the sides of the tin.
Put the prunes, dates, fresh and ground ginger and black treacle in a saucepan with the milk and warm through for about 4 minutes until softened. Take off the heat and beat well to make a paste.
Cream the butter and sugar together until light and fluffy. Beat in the eggs, then beat in the fruit paste. Stir in the flour and bicarbonate of soda. Pour into the tin and bake for 45–50 minutes. Leave in the tin for 5 minutes, then cut into 10 or 12 portions.
Make the sauce while the cake is cooking. Heat the sugar and butter in a small saucepan, stirring to combine. When the butter has melted, turn up the heat and boil rapidly for 5 minutes. Turn down the heat and add the crème fraîche and salt. Simmer for 2 minutes. Leave in the pan and it will thicken as it cools. Warm through gently when ready to serve.
Dark muscovado sugar gives an almost bitter toffee taste; if you prefer, use light muscovado.
Use double cream if you don’t have crème fraîche. Add salt to taste.
Portions of this pudding will keep well for a few days wrapped in cling film or will freeze well too.
The sauce will go solid when chilled. To reheat, warm the pudding through in the oven and melt the sauce in a small pan. Or put a portion of pudding on a plate and top with a dessertspoonful of solid sauce then microwave on high for 30 seconds; the pudding will be warm, with melted sauce.