Cream butter and sugar until pale and fluffy.
Beat in egg and vanilla extract.
Add flour, baking powder (if using), and salt. Mix until just combined.
Roll dough to around 0.5 cm thickness and cut into circles using your circle cutter. Cut out the centres using the larger end of a 2D piping nozzle (or similar) as a cutter.
Chill cookies in fridge for 1 hour.
Preheat oven to 175°C (350°F). Bake for 8–10 minutes until just turning golden. Cool on wire rack.
Make royal icing by putting egg whites (or meringue powder and water) in the bowl of a stand mixer. Add in the sifted icing sugar and lemon juice and beat for 5 minutes, or until thick and glossy.
Colour royal icing with green food colouring. Pipe wreath designs using the small star nozzle, and add festive sprinkles. Leave to set.