Ideally using a stand mixer with the paddle attachment, mix the butter, caster sugar and light brown soft sugar together until fluffy and paler in colour. This will take 3-5 minutes in an electric mixer
Add the egg and vanilla extract and mix in
Add the plain flour, baking powder, bicarbonate of soda and salt and mix them in until just combined
Then mix in the white chocolate chips and sprinkles briefly, just to evenly distribute them
Divide the dough into portions, I weighed the dough out to make each cookie an equal size
Roll the dough portions into balls using your hands, place them on either a lined baking tray, or into a tupperware container lined with baking paper. Chill them in the fridge for 30 minutes
Meanwhile, pre-heat your oven to 180C Fan/200C/400F/Gas Mark 6
After the 30 minutes is up and your oven is pre-heated, place the dough balls onto lined baking trays, leaving space (at least 2") between them to spread. If you don't have enough baking trays, keep the dough in the fridge and bake the cookies in batches
Bake for 12 minutes, remove from the oven and leave the cookies to cool for at least 10 minutes on the baking trays before moving to a cooling rack
Once fully cool, decorate them with melted white chocolate and extra sprinkles. You can pipe the white chocolate on, or drizzle with a spoon. Add the sprinkles while the chocolate is still wet, so they stick to it
Store leftovers in an airtight container and eat within 3 days