Preheat the oven to gas 5, 190°C, fan 170°C. In a small pan, gently heat the butter, golden syrup and sugar, stirring until the butter has melted and the sugar dissolved. Add the stem ginger and 1tbsp syrup from the jar, then add the oats and ground ginger, and stir to coat.
Fold through the dark chocolate, then press the mixture into a greased and lined 15 x 20cm (6 x 8in) baking tin. Bake for 25 minutes, until golden. Cool in the tin before slicing.
You can freeze them in an airtight container, for up to a month. They can be stored in an airtight container in a cool dark place for up to a week.