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 combined.
03 - In a large bowl, beat the butter and sugar together until light and fluffy, approximately 3 to 4 minutes.
04 - Beat in the eggs one at a time, mixing thoroughly after each addition. Stir in the vanilla extract.
05 - Combine the elderflower cordial and milk in a small measuring cup or bowl.
06 - Add the flour mixture to the butter mixture in three additions, alternating with the milk mixture. Begin and end 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 to 20 minutes, or until a toothpick inserted in the center comes out clean.
09 - While the cupcakes bake, combine the elderflower cordial, water, and sugar in a small saucepan. Heat gently over low heat, stirring constantly until the sugar dissolves completely. Remove from heat and let cool.
10 - When the cupcakes are done, let them cool in the pan for 5 minutes. Transfer to a wire rack and brush the warm cupcakes with the elderflower syrup. Allow to cool completely before frosting.
11 - Beat the softened butter until smooth and creamy. Gradually add the powdered sugar, beating until fluffy and well combined. 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.