Beat cool butter and sugar on medium speed for 3 to 4 minutes until pale, fluffy, and noticeably increased in volume. Scrape the bowl halfway through.

Add the egg and vanilla. Beat for 1 minute until smooth and glossy.

Whisk flour and salt together in a separate bowl until evenly combined.

With the mixer on low, add the flour mixture in three additions. Stop immediately when no flour streaks remain — do not overmix.

Divide dough into two flat discs. Wrap in plastic and refrigerate for 1 hour minimum (up to 3 days).

Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.

Roll one disc on a lightly floured surface (or between parchment sheets) to ¼-inch (6 mm) thickness. Cut with chilled metal cutters. Transfer to baking sheet.

Bake on the middle rack for 10 to 12 minutes until edges are pale gold and centres look just barely set. Rest on the sheet 5 minutes, then transfer to a wire rack. Cool completely before decorating.

For royal icing: beat powdered sugar, meringue powder, and 3 tablespoons water on medium-high for 3 minutes until thick and glossy. Thin with additional water one teaspoon at a time. Divide and colour. Outline with 15-second icing, flood with 10-second icing. Allow to dry at room temperature for at least 2 hours (hard-set overnight).