Process the dried hibiscus flowers in a coffee grinder or food processor until fine. Set aside.
In the bowl of an electric mixer, fitted with the paddle attachment, cream butter and sugar at medium speed until well combined, about 2 minutes. Add the egg, lemon zest, 2 teaspoons of hibiscus flower powder, vanilla, and salt. Mix until well combined. Scrape the bowl as needed.
Reduce the speed of the mixer to low and slowly add the flour. Beat until a dough starts to form. Remove the dough from the bowl and shape into a disk. Wrap with plastic wrap and refrigerate for 30 minutes.
Preheat oven to 325°F. Line 2 baking sheets with parchment paper.
Remove the dough from the refrigerator and roll to 1/4-inch thick on a lightly floured surface. Cut dough with a round 3-inch cookie cutter. Place the cookies in the prepared baking sheets and bake for 18 minutes or until golden brown. Remove from oven and allow cooling at room temperature.
While the cookies are cooling down, prepare the glaze by mixing the ingredients for the glaze until smooth. Once the cookies have cooled, glaze and decorate with hibiscus flower powder and lemon zest.