In a large bowl, use an electric mixer to cream the cold butter and powdered sugar until just combined. Mix in the orange zest and vanilla extract.

In a separate bowl, whisk together the flour and salt. On low speed, gradually add the dry mixture to the butter mixture until a crumbly dough forms. Do not overmix.

Turn the dough onto a clean surface and form it into a ball. Divide it in half and roll each portion into a log about 1.5-2 inches in diameter. Wrap the logs tightly in plastic wrap and refrigerate for at least 2 hours until very firm.

Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.

Unwrap the chilled dough and slice it into ¼-inch thick rounds. Place them 1 inch apart on the prepared baking sheet. Bake for 12-15 minutes, until the edges are lightly golden.

Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

Melt the chopped dark chocolate in the microwave in 30-second intervals until smooth. Dip half of each cooled cookie into the chocolate. Place back on the parchment paper and let the chocolate set completely before serving.