Updated Nov 28, 2019Chatelaine
Lemon Royal icing:
1. Stir flour with baking powder and salt in a medium bowl. Beat butter with granulated sugar in a large bowl, using an electric mixer on medium-high, until fluffy, 2 to 3 min. Beat in eggs and lemon zest. Reduce speed to low, then gradually beat in flour until just combined.
2. Divide dough into 2 portions. Shape each into a ball, then flatten into discs and wrap separately in plastic wrap. Refrigerate until firm, at least 1 hr.
3. Position rack in centre of oven, then preheat to 350F. Line 2 baking sheets with parchment. On a lightly floured surface, roll 1 disc of dough until 1/4 in. thick. Cut out shapes with a 2-in. star-shaped cookie cutter. Arrange about 1 in. apart on prepared sheet.
4. Bake until cookies are golden around edges, 10 to 11 min. Let stand on sheet for 3 to 4 min, then transfer to a rack to cool completely. Repeat with remaining disc. Gather dough scraps and form into a disc. Chill, then repeat rolling, cutting and baking process.
5. Icing: Beat egg white with cream of tartar in a large bowl, using an electric mixer on medium, until foamy. Beat in icing sugar until combined. Increase speed to high and beat until thick and glossy, 3 to 4 min. Stir in lemon juice, 1 tsp at a time, to thin the icing out to desired consistency. (Icing should be a thick but pipe-able consistency.) Scoop half the icing into a piping bag fitted with a small round tip.
6. Topping: Pipe lines or squiggles on 1 cookie, then sprinkle immediately with gold sugar. Repeat with remaining cookies. Set aside until icing is completely dry, preferably overnight.
Refrigerate the dough discs up to 1 week. Leave at room temperature for 10 min before rolling out.
Cookies will keep well stored in an airtight container up to 5 days.