Pre-heat oven to 160ºC (140ºC Fan) and grease and line a 900g loaf tin. In a pestle and mortar, combine 150g sugar with the lemon verbena leaves and grind to a pulp. Add to a bowl with the remaining sugar and whisk until fully combined.
Meanwhile, in a large bowl, add butter and most of the sugar (reserve a third for the drizzle) and using a handheld electric whisk, beat on a high speed for 2-3 minutes, until light and fluffy. Reduce speed to a medium, and add your eggs one egg at a time, followed by buttermilk, cordial, baking powder and lemon zest. Fold in flour and pour cake mixture into prepared loaf tin.
Bake in the centre of the oven for 50 to 55 minutes, until golden and a cake skewer poked into the centre comes out clean.
Meanwhile, make the drizzle: combine the reserved lemon verbena sugar with elderflower cordial and lemon juice. Mix until fully incorporated. Poke holes into the warm loaf cake with a skewer and pour over the drizzle. Leave cake in the tin until cooled, a hard sugar crust should now have formed. Slice and serve.