Butterplätzchen (German Cut-Out Christmas Cookies)

Method

1. 1Rub the cold butter into the dry ingredients. Cut the butter into small cubes and add it to a bowl with the flour, caster sugar, lemon zest, vanilla sugar and salt. Work everything together with your fingertips or the paddle of a mixer until the mixture looks like coarse, buttery crumbs and no dry flour is visible at the bottom of the bowl.
2. 2Bring the dough together fast. Add the egg yolk and knead briefly, by hand or on low speed, only until the dough just holds in one piece, 2 to 3 minutes at most. The longer you knead, the more the friction warms the butter, and warm butter gives a greasy dough that spreads flat in the oven instead of holding its cut shape.
3. 3Chill the dough. Flatten it into one or two discs about 2 cm thick, wrap tightly and refrigerate for at least 1 hour, or overnight. Cold, firm dough rolls out cleanly without sticking to the pin, and the rest also relaxes the gluten in the flour so the cut shapes do not shrink back once you lift them off the mat.
4. 4Roll and cut. Heat the oven to 180°C top and bottom heat (160°C fan). Place the dough between two sheets of baking paper and roll it to an even 3 to 4 mm, working in one portion at a time and keeping the rest in the fridge. Rolling between paper instead of on a floured surface keeps the dough from picking up extra flour, which would otherwise toughen the crumb and blunt your cutters. Stamp out shapes close together to limit the scraps, and lift them onto lined trays with a thin spatula.
5. 5Chill the cut shapes. Once a tray is full, put it in the fridge for 10 minutes before it goes in the oven. This short second chill is what keeps a star's points and a bell's curves crisp and defined; skip it and the same shapes soften and blur in the first minutes of heat.
6. 6Glaze half the tray, if using. Whisk the second egg yolk with the milk and brush it thinly over some of the cookies for a deep golden shine after baking; leave the rest of the shapes plain if you plan to ice them later, since icing needs a bare surface to stick to.
7. 7Bake one tray at a time on the middle rack for 8 to 10 minutes. The cookies are done when only the edges turn light gold and the centers are still pale; a fully golden or browned cookie has crossed into a bitter, over-baked shortbread taste. Cool completely on a rack before handling, since warm cookies are fragile and the glaze needs time to set.
8. 8Ice the plain cookies. Whisk the icing sugar with the lemon juice to a thick paste that falls off the whisk in a slow ribbon, adding the juice a few drops at a time so it does not turn runny. Spread or pipe it onto the cooled cookies, scatter sprinkles over the wet icing so they stick, and let everything set at room temperature for at least 4 hours before stacking the cookies in a tin with baking paper between the layers.