Take the butter out of the fridge if you haven't. Preheat the oven at 190 °C and have a piping bag ready and open in a cup.
Line the sheet tray with baking paper or silicon baking mat. Optionally, mark the baking paper with 30 3-cm-diameter circles using a bottle cap or a cookie cutter. First dip the cap to a bowl of flour and stamp it on the baking paper/mat.
Combine 1 Whole Egg, 1 Egg Yolk, and 30 g Sugar in a small mixing bowl and use a hand mixer to beat the mixture until pale, fluffy, and not too liquity. The batter should hold its shape after drawing an “8” on it
Sift in 60 g Cake Flour and GENTLY fold the batter to mix in the flour with a spatula until just combined.
Pour the batter to the piping bag, cut off the tip of the bag, and pipe the batter into 3 cm circles on the baking .
Dust a layer of some Powder Sugar on top of the circles with a sift.
Bake for 10 mins in the preheated oven. Watch closely and take the tray out as soon as the top gets a hint of brownish-orange hue. It might take less more more time.
Peel the shell off the paper with a flat and thin tool like a small knife, offset spetula or bench scraper and let cool. Or you can try bare hand, just peel carefully and don't leave a big chunk of shell on the paper.