How to Make It
Combine the flour, baking powder and salt in a bowl and set aside. Beat the butter and sugar together with an electric mixer. Next, beat in the eggs, one at a time, followed by the zest and citrus juice. Add the flour mixture and beat until combined. Fold in the walnuts and divide the dough in half. Wrap the two dough pieces in foil and stick in the fridge until firm, about 30 minutes.
Preheat the oven to 350° and line a large baking sheet with parchment paper. Remove the dough from the fridge and shape both pieces into logs (measuring about 12 inches long and 3 inches wide) and place a few inches from each other on a prepared baking sheet. Bake for 30 minutes or until golden brown. Remove from oven and transfer to a cooling rack for 15 minutes.
Place hibiscus leaves in the warm water and set aside.
Reduce oven temperature to 300°. Once cooled, use a serrated knife to cut 1/2-inch-thick diagonal slices into the logs. Transfer slices to baking sheet, making sure that you turn the pieces so the cut part is facing up. Bake again until dry and pale golden, about 30 more minutes. Remove from oven and cool completely.
Strain the hibiscus leaves from the water (which should be bright red at this point) and discard the leaves. Place the powdered sugar in a small mixing bowl and slowly whisk in the hibiscus water until a thick glaze forms (this may not take all 2 tablespoons of water, so pour just a little bit of water each time). Drizzle over biscotti and serve with tea or coffee.