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Keep in an airtight container for up to 5 days.
Tip the butter into the bowl of a free-standing mixer, add the caster sugar and beat for 3-4 minutes until pale and light. Add the lemon zest and vanilla, and mix to combine.
Sift the plain flour, rice flour and a pinch of salt into the bowl, and mix again until smooth and combined. Tip the dough out onto a work surface and shape into a neat rectangle, roughly 2cm thick; wrap in clingfilm and chill for at least 4 hours.
Pat the stem ginger dry and chop finely.
Take the dough out of the fridge 20 minutes before you plan on rolling it out to allow it to soften slightly. Lightly dust a work surface with flour, and roll out the dough into a neat rectangle - about 27cm x 18cm. Trim the edges with a long knife to neaten (so it measures about 24cm x 16cm). Cut the shortbread into fingers, each 2cm x 8cm, then scatter over the stem ginger and flaked almonds. Arrange on baking sheets lined with baking paper, leaving a little space between each biscuit, and chill for 30 minutes. Preheat the oven to 170°C, fan 150°C, gas 3.
Sprinkle some caster sugar over the shortbread and bake on the middle shelf of the oven for about 25 minutes until crisp and starting to turn golden brown at the edges.
Leave to cool on the baking sheets for 10 minutes, then transfer to a wire rack and leave until cold.