In a large mixing bowl or the bowl of a stand mixer, add butter, sugar, orange oil, orange juice, and orange zest and mix on high speed until fluffy and well-combined.

Add flour and salt and mix on low until the mixture resembles coarse breadcrumbs.

Use your hands to bring the mixture together, kneading gently until a loose, crumbly dough forms.

Shape dough into a log, approximately 8 inches long and about 1 1/2 inches in diameter.

Wrap the log in clingfilm/plastic wrap and chill in the refrigerator until firm, at least 30 minutes.

While dough chills, preheat oven to 180° C / 355° F. Line a baking tray with parchment or a silicone mat, and set aside until ready to use.

Unwrap the log. With a sharp chef’s knife, slice into 16 slices, each about 1/2 inch wide.

Transfer to prepared tray.

Freeze cookies on tray for 10 minutes. Alternately, you can freeze the slices between squares of parchment, if your tray does not fit in the freezer.

Bake in preheated oven until shortbread is lightly golden around the edges, about 14-15 minutes. (If using two trays, bake one at a time.)

Remove from oven and transfer to a wire rack to cool.

To decorate, line a tray or plate with parchment or a silicone mat.

Melt dark chocolate over a bain marie or double boiler.

Dip one edge of each shortbread cookie in the melted chocolate, and transfer to prepared tray.

Sprinkle with coarse sea salt.

Allow cookies to sit, uncovered, at room temperature until the chocolate sets. Store  in an airtight container at room temperature for up to five days.