Preheat the oven to 170C/ 325F/ GM3 and grease and line the 1lb loaf tin.
Use electric beaters to cream the butter, sugar, syrup and treacle together for around 3 minutes until lighter in colour and fluffy.
Beat in the eggs, one at a time, followed by the soured cream. Don't worry if the mixture splits (curdles) - once the flour is mixed in it will come back together.
Mix the flour, baking powder and ground ginger together and sieve into the bowl, then fold through using a large metal spoon.
Finely dice the stem ginger, reserve 15g for the top and fold the rest through the cake batter.
Spoon into a greased and lined loaf tin and bake the stem ginger cake for 40 minutes (cover the top loosely with parchment after 30 minutes if it looks as if it might burn.
Test the cake is cooked through by poking a clean skewer into the cake – if it comes out clean then the cake is cooked. If there is still wet batter on the stick then let the cake cook for a further 5 minutes and retest.
As soon as the cake comes out of the oven poke holes all over it using a cocktail stick then spoon 3 tablespoons of syrup from the stem ginger jar over the top and scatter the reserved chunks of stem ginger over it.
Let cool completely in the tin then unmold and wrap the ginger loaf cake loosely in foil and store for 24 hours (to allow the flavour to develop) before slicing into it.