Sift icing sugar into butter, mix until combined with spatula, then beat over medium speed until light and fluffy (about 1 minute). Add salt to egg, mix until the salt has dissolved, add egg gradually into butter, beat until well mixed (about 1 minute).
Sift and mix together plain flour, cornstarch and baking powder, sift again into butter. Combine first with spatula, then mix over low speed with mixer until well combined (about 30 seconds). (Reminder: Mix until just combined, don’t overmix to avoid activating the gluten.)
Put a small portion of cookie dough into piping bag with piping tip (with closed star shape). (Reminder: Don’t put too much into piping bag at once as it will be difficult to pipe out. If you don’t have piping bag, just shape the dough into balls and lightly press down with fork to create pattern.)
Pipe cookie dough into round shape (like a flower) onto baking trays lined with silicon baking cloth (or non-stick baking paper).
Keep the piped cookie dough refrigerated for at least 20 minutes, until hardened before baking in oven (this will help the cookie to maintain its pattern when baked, if you bake it straight away after piping, the pattern will not be clear).
Put the hardened cookie dough into preheated oven, bake at 170°C/340°F for 15 minutes (turn the baking tray in between baking to help the cookies bake evenly).
When the baking is done, let the butter cookies cool in the oven for 5 minutes before taking out (this will help the moisture to evaporate completely, the cookies will continue to brown will cooling in the oven). After taking out the baked cookies, preheat the oven again before baking the next batch.
Allow the butter cookies to cool completely before storing in air-tight container.