Preheat the oven to 150°C (300°F) and line two large baking trays with baking paper.
Beat the butter in a stand mixer with the paddle attachment on medium-high speed for 2 minutes until pale and creamy, then add the icing sugar and beat for another 2 minutes until light and fluffy, scraping down the bowl as needed.
125 g unsalted butter, 70 g pure icing sugar
Add the vanilla extract, then sift in the plain flour and rice flour; mix on low speed just until the dough comes together, being careful not to overmix.
1 tsp vanilla extract, 185 g plain flour, 60 g rice flour or cornstarch
Divide the dough into two equal portions and roll each on a lightly floured surface into a log about 4 cm (1½ in) in diameter; wrap in cling film and chill for at least 1 hour until firm.
Slice the chilled logs into 8 mm (¼ in) thick rounds and arrange 2 cm apart on the prepared trays.
Bake for 20–25 minutes, rotating the trays halfway through, until the edges are just golden.
Let the cookies rest on the trays for 10 minutes, then transfer to a wire rack to cool completely.
Notes
During my earliest trials, chilling the dough for just 30 minutes left the rounds spreading too thin, so I always chill for a full hour for clean edges.After one bake, rotating the trays mid-cycle produced perfectly even golden rims every time, a small tweak that made a big difference.In my storage trials, cookies remained crisp after 5 days when kept in a dry, airtight container away from direct light.