Preheat the oven to gas 4, 180°C, fan 160°C. Sift the flour, baking powder and a pinch of salt into a large bowl. In a separate bowl, whisk the sugar and butter with an electric whisk until light and fluffy. Add the elderflower cordial and lemon zest, then gradually add the beaten egg, beating well between each addition. Fold in the flour mix, using a large metal spoon. Add a splash of milk to loosen, if needed.
Line a muffin tin with cupcake cases and divide the mixture between the cases. Bake for 18-20 minutes, until lightly golden and springy to touch, then remove from the tray and leave to cool on a wire rack.
To make the icing, beat the softened butter, icing sugar and cordial in a bowl, until light and fluffy. Add a squeeze of lemon juice, to taste.
Once the cupcakes are completely cool, pipe or spread icing onto each cupcake. Decorate with fresh berries, such as blueberries and raspberries, and sprinkle with lemon zest.