Salted Butter and Chocolate Chunk Shortbread, or Why Would I Make Another Chocolate Chip Cookie Ever Again?

plus 2 tablespoons (255 grams) salted butter (see Note), cut into ½-inch pieces

granulated sugar

light brown sugar

vanilla extract

all-purpose flour

semi- or bittersweet dark chocolate, chopped (but not too fine, you want chunks, not thin shards of chocolate)

large egg (50 grams), beaten

Demerara sugar, for rolling

Flaky sea salt, such as Jacobsen, for sprinkling