Line baking trays with parchment paper. Make sure there is enough space in your refrigerator to place the cookies before baking.
Prepare the piping bags by inserting a large star tip (tip 1M) into a disposable plastic bag. Snip the pointed end. Separately, snip the end of the second piping bag and insert the one with the star tip into it. Set aside.
Add butter, Biscoff cookie butter and salt into a medium sized bowl. Sift in the powdered sugar to break any lumps in the sugar.
Beat all the ingredients until creamy. Scrape the sides of the bowl.
Add the egg. Beat again until well incorporated.
Add vanilla extract and beat again.
Sift in the cornstarch. Mix well.
Sift in the all purpose flour.
Mix with a spatula or a cake mixer until the flour is well combined. The dough should be very soft at this stage.
Transfer one third of the dough into the prepared piping bag.
Pipe small rosettes onto the prepared baking trays. The cookies will spread during baking so make sure to space them out on the tray.
Place the piped cookies along with the baking tray into the fridge for 30 minutes. Meanwhile, pre heat the oven to 170°C.
Place the chilled cookies into the preheated oven and bake for about 14 to 15 minutes.
Remove the cookies from the oven and let them rest in the tray for a few minutes before transferring them onto a wire rack to cool completely.
Store the cookies in an airtight container.