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