Line 3 large baking trays with baking parchment. Put the butter and caster sugar in a bowl and whisk until pale and creamy. Whisk in the egg yolk and lemon rind
Sieve over the flour and Baking Powder and carefully mix together to make a mixture that resembles a crumble topping. Using your hands, bring the mixture together to form a firm dough. Turn on to a lightly floured surface and knead until smooth and pliable.
Roll out the dough to a thickness no greater than 1/2cm (1/4inch). Using a 6cm (2 1/2inch) wide heart shaped cutter stamp out approx. 40 cookies, re-rolling the dough as necessary. Arrange on the prepared baking trays, prick lightly with a fork and chill for 30 minutes.
Ten minutes before baking, preheat the oven to 180°C (160°C Fan, Gas Mark 4). Bake the cookies for about 10 minutes until firm and lightly golden round the edges. Leave to cool on the baking trays for 10 minutes before transferring to a wire rack to cool completely.
To ice, make up the Egg White Powder using 30ml (2 tbsp) warm water and the lemon juice as directed on the packet. Sift the icing sugar into a bowl. Add the blended lemony egg white.
Carefully combine the wet and dry ingredients together to make a soft, spreadable icing – if it is too stiff add a few drops of lemon juice, or if too dry, add some more sifted icing sugar, 15ml (1 tbsp) at a time. Beat until smooth and glossy.
Spoon approx. 30ml (2 tbsp) of the icing into a small piping bag fitted with a No.2 size plain piping nozzle. Pipe a line approx. 1mm from the edge of all of the cookies.
Next, working on one heart at a time, spoon approx. 5ml (1tsp) of the icing into the centre and gently spread it to fill the centre, tapping gently to make an even layer and release any air bubbles. Add a little more icing if necessary in order to achieve a smooth, well-covered surface.
Decorate with Soft Silver Pearls and place back on the wire rack whilst you ice the remaining cookies.
The icing will be firm enough to serve after 1 hour. For a firmer set, leave the icing to dry for 4 hours – this icing will not dry out to a crisp.