Grease and line the base and sides of a standard 1lb loaf tin with baking parchment, preheat the oven to 180C/Fan 160C/Gas 4.
Place the butter and sugar in the bowl of a stand mixer fitted with a paddle attachment and beat them together until they are light and fluffy.
Slowly add the eggs and beat them in between each addition letting them mix in completely.
Add the lemon zest and lavender and beat them in slowly. Combine the flour and baking powder and sift them into the beaten egg mixture. Slowly beat in the flour, be careful not to overmix the cake.
Pour the mixture into the loaf tin and using a dinner knife run a line the length of the cake straight down the middle, touching the base of the tin. This will help the cake rise with a nice even line down the middle.
Bake for 45 minutes until the cake is golden and a cocktail stick comes out clean.
While the cake is cooking, make the syrup by gently heating the caster sugar and lemon juice together in a saucepan until the sugar is completely dissolved.
When the cake is cooked remove it from the oven and pierce it all over with a cocktail stick, take the syrup and pour it all over the cake so that it seeps in. Leave the cake to cool in its tin.
When the cake is completely cooled, make the icing by mixing the icing sugar with the lemon juice until you have a pourable icing. Add more lemon juice if necessary.
Drizzle the icing over the cake and then sprinkle the top with some more of the lavender. The cake is ready to serve.