01 - Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with cupcake liners.
02 - In a medium bowl, whisk together the flour, baking powder, and salt until evenly distributed.
03 - In a large bowl, cream the softened butter and granulated sugar together until light and fluffy, approximately 3 minutes.
04 - Beat in the eggs one at a time, mixing thoroughly after each addition. Stir in the vanilla extract until incorporated.
05 - Mix the elderflower cordial and room temperature milk together in a small measuring cup.
06 - Add the flour mixture to the butter mixture in three additions, alternating with the milk-cordial mixture. Begin and end with the flour, mixing until just combined. Avoid overmixing.
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 granulated sugar in a small saucepan. Heat gently over medium-low heat, stirring constantly until the sugar dissolves completely. Remove from heat and let cool.
10 - When the cupcakes are done, cool in the pan for 5 minutes, then transfer to a wire cooling rack. Brush the warm cupcakes evenly with the elderflower syrup. Allow to cool completely before frosting.
11 - Beat the softened butter until smooth and creamy. Gradually add the sifted 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 completely cooled cupcakes with the elderflower buttercream. Decorate with edible flowers or a sprinkle of lemon zest if desired.