In a large mixing bowl, beat 1 cup of softened salted butter with 1/3 cup granulated sugar and 1/4 cup light brown sugar until the mixture is light and fluffy, about 5 minutes. Mix in 1 teaspoon of vanilla extract until well combined. Gradually add 2 and 1/4 cups of all-purpose flour on low speed until no streaks remain, then fold in 1 cup of dark chocolate chips and 2 teaspoons of fresh orange zest, ensuring an even distribution.

Once your dough for the Dark Chocolate Orange Shortbread Cookies is well mixed, divide it into two equal halves. Shape each half into a log, using plastic wrap to keep the shape tight and compact. Refrigerate the wrapped logs for at least 2 hours or overnight.

Preheat your oven to 350°F (177°C). Line two baking sheets with parchment paper for easy clean-up.

After chilling, brush each log of dough gently with an egg wash made from 1 beaten egg. Roll the logs in turbinado sugar until evenly coated. Slice the logs into ½-inch thick rounds and arrange them on the prepared baking sheets.

Bake the cookies for 12-14 minutes, or until the edges are just golden brown. Let the cookies cool on the pan for 10 minutes before transferring them to a wire rack.

Optionally garnish the cookies with extra orange zest and a sprinkle of flaky sea salt before serving.