Prepare two baking sheets lined with parchment paper and a piping bag fitted with an open star tip. I use the Ateco 827 nozzle (for cookies with sharp edges) or the Willton M1 piping nozzle for rose-like shaped cookies.
In a bowl, add butter, granulated sugar, powdered sugar, and vanilla extract. Beat with a hand mixer until light and fluffy.
Add the egg and beat until well incorporated.
Sift in flour and salt and mix on low just until no dry ingredients are left. You can also combine everything together using a rubber spatula.
Place the dough into the piping bag. Pipe 30 rosettes around 1½ ” (4cm) in diameter leaving 1” in between each rosette. Freeze the cookies for 20-30 minutes.
Meanwhile, preheat the oven to 375F (190C).
Bake one batch of cookies at a time for 10 minutes or until they start picking up a golden color around the edges. The tops of the cookies should stay pale.
Remove from the oven and allow to cool down on the tray for 10 minutes. Then, transfer onto a cooling rack to cool down completely.