1) Whisk the flour, baking powder and salt in a medium bowl. Beat the butter and both sugars in another medium bowl with an electric mixer on medium-high speed until light and fluffy, about thirty seconds. Add the egg yolks, vanilla and orange zest mixing until fully incorporated. Slowly add the flour mixture, and continue beating until the dough comes together, stopping and scraping down the sides of the bowl as needed.
2) For rolled biscuits: Roll about a tablespoon of dough by hand into a ball. Dip one side of the ball into some coarse sugar and place them sugar-side-up on an ungreased baking sheet, leaving about 2.5cm between biscuits.
3) For sliced biscuits: Divide the dough in half, roll by hand into 5cm-wide logs, wrap in cling film and refrigerate until firm, at least two hours. Cut the logs into 0.5cm-thick biscuits and place them on ungreased baking sheets, leaving about 2.5cm between biscuits.
4) For cutout biscuits: Divide the dough in half, pat into disks, wrap in cling film and refrigerate until firm, at least two hours. Roll the dough between lightly floured parchment, or waxed paper, until about 1cm thick. Transfer the sheets to a baking sheet and refrigerate until firm, about 30 minutes. Cut into desired shape using a biscuit cutter and place on ungreased baking sheets, leaving about 2.5cm between biscuits. (Gather the dough scraps together, pat into a disk, chill and reroll.)
5) Refrigerate the biscuits while preheating the oven to 190°C / gas mark 5, for at least 30 mins. Bake the biscuits until the bottoms are golden, about 10 to 15 mins depending on shape. Cool on sheets until firm enough to transfer to a rack to cool. Decorate as desired and serve, or store in an airtight container at room temperature for up to one month.
For the royal icing:
1) Combine all the ingredients, except the food colouring, in the bowl of a standing mixer fitted with the paddle attachment. Mix slowly until stiff enough to form peaks. The icing should be pure white and thick, but not fluffy and bubbly. If the frosting is overbeaten, it will get aerated which makes it harder to work with. If this happens, let the frosting sit to settle, then use a rubber spatula to vigourously beat and smooth out the frosting.
2) Alternatively, combine the ingredients in a large bowl and beat with hand beaters on low speed until the frosting thickens to stiff peaks.
3) Add up to one tablespoon of food colouring and mix with a rubber spatula until the colour is uniform. (Adding too much colour reduces the sheen of the frosting and can break down the consistency of the frosting over a couple of days.) Store the icing covered, with cling film on the surface to prevent it hardening.