Grind strawberries and hibiscus flower separately in a spice grinder until they are a fine powder. You may have to do this in batches depending on the size of your grinder.

Whisk ground strawberries and hibiscus flowers with the flour until evenly mixed; set aside.

In a medium bowl, with the back of a large spoon or with an electric mixer, beat butter with sugar and salt for about 1 minute, until smooth and creamy but not fluffy. Add flour mixture and mix just until incorporated.

Turn off the mixer, scrape down the bowl, form dough into a ball, and, if necessary, knead it with your hands a few times until smooth. Form into a 12-by-2-inch log. Wrap and refrigerate for at least 2 hours or overnight.

Heat the oven to 350°F and arrange the racks in the lower and upper third. Use a sharp knife to cut the cold dough log into 1/4-inch-thick slices. Place cookies at least 1 1/2 inches apart on ungreased baking sheets.

Bake cookies until light golden brown at the edges, about 12 to 14 minutes, rotating the baking sheets from top to bottom and front to back halfway through baking. Remove from the oven and let cookies firm up on the pans for about 1 minute, then transfer them to a rack and let cool completely.