Beat the butter in a large bowl until creamy. Add the sugar and lemon zest and mix well. Add eggs and beat until incorporated. Sift in half of the flour and beat until incorporated, then repeat with the remaining flour.
Gently knead the dough in the bowl until it holds together as a ball. Transfer the dough to a clean work surface and briefly knead again. Shape the dough into a thick rectangle and wrap well in cling film. Chill for 2 hours.
Preheat oven to 180 C / Gas 4. Line baking trays with baking parchment.
Roll dough out thinly on a floured surface, to a thickness of about 5mm. Cut out heart shapes then transfer to prepared baking trays.
Bake in preheated oven for 10 to 15 minutes (after the first tray, the baking time is shorter) until golden brown. Remove from the oven and let cool completely before decorating.
For the flood technique, use runny royal icing in a piping bag with a small, round #2 tip. See tip below.
Pipe a line around the edge then fill in the centre. Evenly distribute the icing with scriber tool.
For hearts, add dots of a different colour onto wet icing then drag scriber tool downwards through dots.
For lips, add an oval shape onto wet icing use scriber tool to shape into lips.
Pipe a line around the edge then add stripes and turn into feathers or patterns.
Or, add little dots or turn them into hearts.
To create a marble effect, pipe lines of different colours across a base layer then drag the scriber up and down through the lines.
Allow the icing to set hard (this can be helped along by placing in a low oven for about 10 to 15 minutes) and then enjoy with someone you love!
To use the excess dough after cutting out the hearts, gather the excess pieces, knead them briefly and roll them out and cut them out as described above.
To make the royal icing
Sift 375g icing sugar into a bowl. Into a separate bowl whisk 2 eggs whites (about 60g) until they become frothy. Add the icing sugar to the egg whites, a spoonful at a time until the mixture becomes very stiff and stands up in peaks. If desired, you can also add a teaspoon or two of lemon juice to thin the mixture out. For an eggless royal icing version, use 375g royal icing sugar with about 40 to 50ml lemon juice (more or less depending on the consistency you desire).