Cut four squares of parchment paper about 15 cm (6 in) across. Lightly oil the flat base of a heavy mug and one side of a metal cookie cutter. Set them next to the stove with a needle or toothpick.
Put 1 tbsp (12 g) sugar in a long-handled stainless steel ladle. Set it over low heat.
Stir slowly with a wooden chopstick or bamboo skewer as the sugar melts. It goes from grainy to cloudy to clear. Stop when it is a clear syrup just turning pale gold, about 1 to 2 minutes — roughly 175°C / 350°F on a thermometer.
Take the ladle off the heat. This matters: leave it on and the next step burns.
Add a scant pinch of baking soda, about 1/8 tsp. Stir hard for 5 to 10 seconds. The syrup turns pale, thickens and rises up the ladle like foam.
Pour it onto a parchment square in one motion. Wait about 10 seconds.
Press it flat with the oiled base of the mug to a disc roughly 10 cm (4 in) wide and 5 mm thick.
Immediately press the oiled cookie cutter into the center, about halfway through the candy. Do not cut all the way to the parchment.
Lift the cutter straight up and leave the disc alone for 3 minutes to harden. Wash the ladle in hot water and repeat for the next cookie.