01 -
In a large bowl or stand mixer fitted with the paddle attachment, beat the butter and powdered sugar on medium-high speed for 3-4 minutes until pale and fluffy. Scrape down the bowl once or twice.
02 -
Add the egg yolk and vanilla extract. Mix on low speed until just combined.
03 -
In a separate bowl, whisk together the flour, cornstarch, and salt. Add to the butter mixture and mix on low until a soft dough forms with no dry streaks.
04 -
Shape the dough into a disk and wrap tightly in plastic wrap. Refrigerate for at least 1 hour, or up to 3 days.
05 -
Pipe rosettes onto parchment-lined baking sheets using a large open star tip, leaving 1 inch between cookies. Alternatively, roll the dough into a log, coat in sanding sugar if desired, and slice into 1/4-inch rounds.
06 -
Bake at 325°F (165°C) for 12-14 minutes, until the edges are just barely golden. Cool on the baking sheet for 15 minutes, then transfer to a wire rack to cool completely.