01 - Preheat the oven to 350°F. Line a 12-cup muffin tin with cupcake liners.
02 - In a medium bowl, whisk together the flour, baking powder, and salt until well blended.
03 - In a large bowl, cream the butter and sugar together until light and fluffy, approximately 3-4 minutes.
04 - Beat in the eggs one at a time, mixing thoroughly after each addition. Stir in the vanilla extract.
05 - Mix the elderflower cordial and milk together in a small measuring cup.
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, being careful not to overmix.
07 - Divide the batter evenly among the cupcake liners, filling each about two-thirds full.
08 - Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean.
09 - While the cupcakes bake, combine the cordial, water, and sugar in a small saucepan. Heat gently, stirring constantly, until the sugar completely dissolves. Remove from heat and allow to cool.
10 - When the cupcakes are done, cool in the pan for 5 minutes, then transfer to a wire rack. Brush the warm cupcakes generously with the elderflower syrup. Let cool completely before frosting.
11 - Beat the butter until smooth and creamy. Gradually add the powdered sugar, beating until fluffy and well incorporated. Beat in the elderflower cordial. Add milk as needed to achieve a smooth, spreadable consistency.
12 - Frost the cooled cupcakes with the elderflower buttercream using a piping bag or spatula. Decorate with edible flowers or a sprinkle of lemon zest if desired.