- BEAT butter and sugar in a large bowl with an electric mixer on medium, scraping sides and bottom of bowl, until fluffy, about 3 min. Beat in yolks, 1 at a time. Beat in salt and vanilla. Sift flour over butter mixture and beat on low until just combined.
- KNEAD on work surface until smooth, about 10 times. Divide dough in half. Shape one half into a flat square on a piece of parchment. Roll out to a 9-in. square, dusting lightly with flour. Slide paper and dough onto a baking sheet, then repeat with remaining dough. Chill until firm, about 25 min.
- PREHEAT oven to 325F. Line 2 baking sheets with parchment.
- TRIM edges of dough to make a perfect square, then cut into 16 2-in. squares. Transfer to a prepared sheet.
- BAKE until edges begin to brown, 14 to 16 min. Let cool for 3 min, then transfer to a rack to cool. Repeat with remaining dough.
- ROYAL ICING: Beat egg whites and vinegar in a large bowl with an electric mixer on medium until foamy. Beat in sugar until combined, then increase speed to medium-high and beat until thick, 2 min. Beat in cold water to thin icing.
- TRANSFER to a piping bag fitted with a very small plain tip. Pipe a border of icing around outer edges of cookie, then continue in concentric circles to the centre. Spread icing evenly with a skewer. Ice all cookies, then let sit at room temperature until icing hardens, about 8 hours.
- STIR ½ tsp lustre dust and ½ tsp water in a small bowl, mixing with a paintbrush. Paint cookies as desired.
Kitchen Tip: Lustre dust is an edible powder used to add metallic sheen and colour. When mixed with water or clear alcohol like vodka, it can be used as a paint on royal icing or fondant. We found lustre dust at goldaskitchen.com. Metallic food paint would also work.