1. Preheat oven to 350 degrees F (175°C). Line a cookie sheet with parchment paper and set it aside.

2. In a large mixing bowl, combine the softened unsalted butter, powdered sugar, and unsweetened cocoa powder. Mix with an electric mixer on medium speed until well blended and creamy. This helps prevent streaks and ensures even chocolate distribution.

3. Add the all-purpose flour, kosher salt, and pure vanilla extract to the butter mixture. Mix on low speed until the mixture becomes crumbly. Do not overmix.

4. Once crumbly, use your hands to finish mixing and bring the dough together. It will seem dry at first, but the warmth from your hands will help the butter incorporate, forming a cohesive ball.

5. Roll the dough into a log shape, about 1.5 to 2 inches in diameter. Using a sharp knife, slice the dough into ¼-inch thick rounds. Place the sliced cookies on the prepared parchment-lined cookie sheet.

6. Bake for about 10 minutes, or until the edges are just set and the cookies are lightly golden. They will still be soft when removed from the oven.

7. Remove the cookie sheet from the oven and let the cookies cool on the sheet for 5 minutes before carefully transferring them to a wire rack to cool completely. Cooling completely is crucial for them to achieve their characteristic melt-in-your-mouth shortbread texture.

8. If desired, melt the Ghirardelli dark chocolate melting wafers in a microwave-safe bowl. Heat in 30-second intervals, stirring after each, until smooth and fully melted (about 1 minute total). Spoon the melted chocolate into a small ziploc bag, snip off a tiny corner, and drizzle artistically over the cooled cookies. Add sprinkles immediately after drizzling, if using.

9. Serve immediately or store in an airtight container for up to 5 days, or freeze for up to 3 months.