Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Grease a 20 cm/8 in square cake tin and line it with baking parchment making sure that you leave some overhanging so that you can easily lift the cake out later.
Whisk together the flour, cornflour, ground almonds, baking powder, bicarbonate of soda, xanthan gum, salt and sugar in a large bowl.
Add the lemon zest and juice, milk, yogurt, melted butter or oil and lemon extract and stir with a balloon whisk until smooth.
Pour the batter into the prepared tin and spread it level. Bake for 30-40 minutes until a skewer inserted into the centre comes out clean.
While the cake is baking, mix together the lemon juice and caster/granulated sugar to form a syrup.
Allow the cake to cool in the tin for 5 minutes then poke holes all over the top with a cocktail stick. Drizzle over the syrup, making sure that you pour it around the edges as well, not just in the middle. Leave the cake to cool in the tin.
Once the cake is cool, carefully lift it out of the tin using the overhanging baking parchment
Sift the icing sugar into a bowl and gradually stir in enough lemon juice to form a thick but pourable icing. Drizzle the icing over the top of the cake and serve. Store any leftovers in an airtight container for up to 3 days.