Preheat the oven to 350 degrees F (175 degrees C). Line a 12-cup cupcake tin with cupcake liners.

Whisk flour, chopped lavender, baking powder, and salt together in a bowl. Set aside.

Beat sugar and butter together in a bowl using an electric mixer on medium speed until light and fluffy, about 3 minutes. Beat in egg, lemon juice, and lemon zest.

Beat 1/3 cup of flour mixture into lemon mixture until just combined. Beat in 1/4 cup buttermilk, followed by another 1/3 cup flour mixture; add remaining buttermilk and flour mixture and stir to combine.

Spoon batter into the cupcake liners, filling each 2/3 full.

Bake in the preheated oven until a toothpick inserted into the center of a cupcake comes out clean, 16 to 18 minutes. Let cool 5 minutes before transferring cupcakes to a wire rack to cool completely.

Stir confectioners' sugar and lemon juice together in a bowl until smooth. Stir in lemon zest. Drizzle over cooled cupcakes and garnish with lavender.