Preheat the oven to 190ºC/375ºF/gas 5. Grease a square cake tin (roughly 20cm) with the margarine, then line the bottom with greaseproof paper and dust the sides with gluten-free flour.
In a bowl, beat the margarine and sugar for around 5 minutes, or until light and fluffy. Crack in the eggs, add the zest from 2 lemons and 1 orange, then mix to combine. Sieve the flour, baking powder and xanthan gum into the bowl, then fold through. Stir in the juice from 1 lemon and ½ an orange until you have a nice, smooth mixture.
Pour the mixture into the cake tin, then place on the middle shelf of the hot oven for 40 minutes, or until golden and an inserted skewer comes out clean. Leave to cool for 5 minutes, before turning the cakes out onto a wire cooling rack, then leave to cool completely while you make the icing.
Combine the icing sugar with the zest and juice from ½ a lemon and ¼ of an orange. Once the cake has cooled, drizzle the icing on top, grate over a little lemon and orange zest, then serve.
To make lemon drizzle cupcakes, simple divide the mixture between two 12-hole muffin trays and bake for 15 to 20 minutes, or until golden and an inserted skewer comes out clean.
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