In a large bowl, whisk together plain flour, baking powder, and salt. In a mixer, beat softened butter for 2 minutes until pale and fluffy. Gradually add caster sugar, then mix in the egg and vanilla. Slowly combine the flour mixture until blended.

Divide the dough into two portions, rolling each between parchment paper to about 5mm thickness. Wrap tightly and refrigerate for 1-2 hours, or ideally overnight, to prevent spreading.

Preheat your oven to 180°C (or 160°C for fan ovens). Cut shapes from the chilled dough and transfer them to lined baking trays. Bake for 10 minutes, rotating trays halfway through, until edges are lightly golden. Cool on trays for 5 minutes before transferring to a wire rack.

In a bowl, beat together royal icing sugar and water until smooth. Adjust consistency by adding more icing sugar or water as necessary. Use gel food coloring to tint your icing in vibrant pastels.

Outline each cookie with royal icing and allow to dry slightly before flooding the center with more icing for smooth coverage. Let the cookies dry completely before adding any additional details or layers.