Preheat the oven to a gas mark 4 or 180°c. Pre-line a muffin tray with cupcake liners.
Combine Flour, Baking Powder, Salt and Lemon Zest into a mixing bowl. Mix and set a side.
In a spearate mixing bowl, begin mixing the butter. When soft and fluffy, add in the sugar and mix until fully combined.
Gradually begin adding in the dry ingredients to the butter/sugar mixture. Alternate adding in the dry ingredients with the eggs. Mix on a low speed until all is combined.
Add cupcake batter to the cupcakes, filling these 3/4 full. Tap the tray gently to release any air bubbles. Bake on a lower shelf on a gas mark 3 for 20-25 minutes, until the tops are golden brown and a skewer comes out clean.
Allow to cool for 5 minutes, then using a skewer poke several holes into the cupcakes. Then pour some of the lemon and elderflower syrup onto the tops of the cupcakes and allow to absorb in. (see below for syrup recipe).
Cut out the centre of the cupcakes, making sure not to cut all the way through. Keep the removed piece of cupcake. Pipe in the lemon filling (see below for recipe) and replace the removed piece of cupcake.
Pipe on the buttercream to finish. Use extra lemon zest to garnish, as well as any leftover lemon filling.