Beat the softened butter and powdered sugar in a large bowl for 2 minutes, until pale and fluffy.
Add the vanilla and salt. Mix for 30 seconds.
Add the flour and cornstarch. Mix on low speed until the dough looks crumbly.
Add the chopped cranberries and press the dough into one soft ball.
Roll the dough into a log about 2 inches wide and 8 inches long. Wrap it tight in parchment paper.
Chill the log in the fridge for at least 1 hour.
Heat the oven to 350°F (175°C). Line two baking sheets with parchment paper. Roll the log in sparkling sugar if you like, then cut it into 1/3-inch rounds.
Place the slices 1 inch apart on the baking sheets. Bake for 12 to 14 minutes, until the edges are just golden.
Cool on the sheet for 5 minutes, then move to a wire rack.