1. Preheat the oven to 325 degrees F or 165 degrees C and line two baking sheets with parchment paper. Prepare a pastry bag with a medium-sized star tip. 

2. In a medium bowl cream the butter with a whisk until smooth and creamy. Mix in the salt. Sift the powdered sugar into the bowl and whisk until thoroughly combined. Add the egg white and mix until incorporated. 

3. Switching to a rubber spatula, sift half the flour into the butter mixture and combine. Once almost all the flour has been incorporated, sift in the rest of the flour. Then add the mugwort powder and mix until combined.   

4. Transfer the cookie dough into the prepared pastry bag. Pipe the dough into desired shapes and swirls, leaving about 1.5 inches or 3.75 cm between each cookie. 

5. Bake for about 12 minutes or until the bottoms have started to turn golden. Remember to rotate baking sheets halfway through baking. Immediately transfer cookies to a cooling rack. 

6. Once cooled, dip in melted white chocolate and sprinkles if using. Store in an airtight container at room temperature for 5 days.