Heat oven to 170C/150C fan/gas 3. Butter a deep, loose-bottomed, 18cm cake tin and line the base with baking parchment.
Beat the butter and caster sugar together with an electric whisk until fluffy and pale in colour. Crack the eggs in one at a time, beating well after each addition, then stir in the lemon zest. Fold in the flour, baking powder and ½ tsp salt, then fold in the yogurt.
Spoon the mixture into the lined tin, smoothing the top with a spatula. Bake in the centre of the oven for 50-55 mins, or until golden brown on top and firm to the touch. Cool in the tin for 10 mins before turning out onto a wire rack to cool completely. Will keep in an airtight container for up to four days, or in the freezer for up to a month.
When you’re ready to decorate, sieve the icing sugar into a bowl and beat in enough of the lemon juice to create a thick icing. Set the cake on a serving plate or cake stand, then spoon over the icing, allowing it to drip down the sides a little. Top with the lemon zest or candied peel, if using (see below), and cut into generous wedges to serve.
To make candied lemon peel, peel large, wide strips from the lemons using a vegetable peeler. Remove any pith with a knife, then julienne the peel into very thin matchsticks. Tip the granulated sugar into a saucepan with 200ml water and set over a medium heat until the sugar has dissolved. Add the lemon peel and simmer gently for 15 mins, then scoop the peel out using a slotted spoon and set on a piece of kitchen paper to cool. Use to decorate the top of your cake.