3) Beat the butter and sugar together in a large bowl with an electric mixer on medium-high speed until just combined and lightly fluffy. Beat in the treacle and egg yolk. Gradually add the dry ingredients and mix together on low speed until dough is just combined and still crumbly, about three minutes. Give dough a few turns with a spatula to bring together.
4) Lay out wax paper on a clean work surface and put about a third of the batter on top. Lightly press down and top with another sheet of wax paper. Using your hands or a rolling pin, gently pat into a rectangle. Roll out with a rolling pin until dough is as thin as possible without breaking, no thicker than 1/5cm thick. This is the key to these cookies: they really can't be too thin. Gently peel back the layer of waxed paper and then replace it loosely.
5) Transfer rolled batter to a flat baking sheet and freeze until firm and can easily be peeled away from the waxed paper, about 30 mins. Repeat with the remaining dough.
6) Cut dough using a small (5 to 7 1/2cm) fluted round cookie cutter and return to the freezer for 15 mins to set. Transfer frozen cookies to a baking sheet lined with parchment paper and bake until crisp and lightly, evenly coloured (but not brown), about ten mins.
Busy baker's tips: Store baked cookies in an airtight container for up to ten days. The dough can be frozen, between sheets of waxed paper and well-wrapped in cling film, for up to two weeks. Baked cookies can be wrapped in cling film then aluminium foil and frozen for up to one month.
Cook's note: Don't be intimidated by how many cookies this recipe makes. This dough freezes well, and you can roll out far in advance -- so when you need cookies, you can cut and bake as needed, which is a great holiday timesaver.