Heat the oven to 350°F (175°C). Line a 13 x 9-inch baking pan with foil or parchment, leaving an overhang on the sides for lifting out the bars, and lightly grease the lining.

Beat the softened butter and packed light brown sugar until smooth and creamy. Add the egg yolk and vanilla extract, then beat until evenly combined.

Gradually mix in the all-purpose flour and sea salt just until a soft dough forms. Avoid overmixing, since too much mixing can make the shortbread less tender.

Press the dough firmly and evenly into the prepared pan, paying special attention to the corners. Bake for 15–20 minutes, until the edges and surface are lightly golden and the center is set but still slightly soft.

Immediately scatter the semisweet chocolate chips over the hot crust. Let them stand for about 3–5 minutes so they soften, then spread the chocolate into an even layer; if needed, return the pan to the warm oven for 1–2 minutes.

Sprinkle the chopped pecans and remaining sea salt over the melted chocolate. Add a little extra flaky salt if desired for a more pronounced salty finish.

Cool the bars completely in the pan for at least 45–60 minutes before lifting them out with the foil or parchment overhang. For cleaner cuts, chill the fully cooled slab briefly, then cut it into about 24 bars with a sharp knife.