• Pre-heat the oven to 180°C/160°C fan/gas mark 4.
• Line and grease an 8 inch square baking tin.
• Cream the butter and sugar with the lemon zest for a few minutes until light and fluffy.
• Add the eggs one at a time, mixing until thoroughly incorporated and stir in the lemon juice.
• Sift together the flours, ground ginger, baking powder and salt then mix into the rest of the ingredients until just combined.
• Stir in the stem ginger.
• Pour the batter into the baking tin, smoothing the top and bake in the oven for around 30-35 minutes until the sponge has browned on top and an inserted toothpick comes out clean.
• Remove from oven, rest for five minutes then carefully turn out onto a wire rack. Either leave the cake to cool completely to reheat later or serve whilst still warm with the custard. Arrange the crystallised ginger over the top of the pudding if using.

Lemon Custard

• Slice the peel away from the lemon half with a very sharp knife, making sure to remove all the pith.
• Place the lemon peel into a saucepan along with the milk and heat until just below boiling point.
• Remove the milk from the heat and leave the milk to infuse with the lemon for at least 2 hours.
• Whisk the egg yolks with the sugar in a medium bowl until pale and thick then stir in the tapioca flour if using.
• Remove the lemon peel from the milk and re-heat the milk in a medium sized saucepan until just below boiling.
• Take the milk off the heat and then pour in a very thin and steady stream into the eggs, whisking all the while until all the milk has been incorporated.
• Pour the custard mixture back into the saucepan and bring to a very gentle boil and the custard has thickened, stirring all the time.
• Remove from the heat and serve over the Lemon and Ginger Pudding.