Line a 30cm springform tin with greaseproof paper. Preheat the oven to 150C/300F/gas 2.
Beat the softened butter in a bowl. Add the muscovado sugar and beat for 4 minutes. On a gentle speed, add one egg yolk at a time until all are included.
Pour in the treacle. Tip in the flour, the spices, the fresh and crystallised gingers, and the ginger syrup. Mix deftly, but swiftly.
Now, add the coffee, then the baking powder and mix.
In a separate bowl, beat the egg whites until stiff, then add the caster sugar, beating all the while until peaked. Fold ⅓ of the egg white mix into the batter, then ⅓ of the sour cream, and continue thus until all is mixed swiftly and deftly.
Pour the batter into the prepared cake tin. Smooth the surface. Put the cake in the oven and bake for 1 hour. Check with a skewer inserted within for doneness. If longer is required then check every 5 minutes or so.
Cool the cake and store in an airtight container somewhere cool – for a fair few days, if possible, before deciding on whether to serve with custard, cream or ice-cream – or all three, perhaps.