01 - Preheat the oven to 350°F. Line a 12-cup muffin tin with cupcake liners.
02 - Whisk together the flour, baking powder, and salt in a medium bowl.
03 - In a large bowl, cream the butter and sugar together until light and fluffy.
04 - Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
05 - Mix the elderflower cordial and milk together in a small bowl.
06 - Add the flour mixture to the butter mixture in three additions, alternating with the milk mixture, beginning and ending with the flour. Mix until just combined.
07 - Divide the batter evenly among the cupcake liners, filling each about two-thirds full.
08 - Bake for 18 to 20 minutes, or until a toothpick inserted in the center comes out clean.
09 - Combine the cordial, water, and sugar in a small saucepan. Heat gently, stirring, until the sugar dissolves. Remove from heat and let cool.
10 - Cool the cupcakes in the pan for 5 minutes, then transfer to a wire rack. Brush the warm cupcakes with the elderflower syrup. Let cool completely.
11 - Beat the butter until smooth. Gradually add the powdered sugar, beating until fluffy. Beat in the elderflower cordial. Add milk as needed to achieve a smooth, spreadable consistency.
12 - Frost the cooled cupcakes with the elderflower buttercream. Decorate with edible flowers or lemon zest if desired.