Cream the butter, sugar, and orange zest on medium speed in the bowl of a stand mixer fitted with the paddle attachment.

Add the sourdough starter, salt and vanilla extract and continue to mix until fully combined.

Slowly add the flour with the mixer set to low speed until just fully combined.

Transfer the dough to a clean work surface. Shape the dough into a smooth cylinder measuring approximately 14 inches long.

Pour the remaining 3 tablespoons of sugar along the long side of the shortbread dough on the work surface. Roll the dough over the sugar to completely coat it on all sides.

Cut the dough cylinder in half, creating two equally sized cylinders, and wrap the pieces in plastic wrap. Chill in the refrigerator for 1 hour or up to 48 hours for a long fermentation.

Preheat the oven to 375℉.

Use a sharp kitchen knife to cut the dough into approximate ⅓ inch cookies.

Place the cookies on a baking sheet lined with parchment paper, spacing them about 1 inch apart.

Bake for 12-15 minutes or until the edges are just starting to turn golden brown.

Allow to cool in the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.

Once the cookies have fully cooled, melt the dark chocolate in the microwave. Heat at 15 second intervals, stirring between each interval, until the chocolate is smooth and fully melted. Do not overheat or the chocolate may burn or seize.

Dip half of each shortbread cookie in the chocolate, then return to the wire cooling rack. Let the chocolate mostly cool before sprinkling with garnishes.

Sprinkle the chocolate with orange zest or flaky sea salt if using. Allow to the chocolate to set completely before serving.