Preheat the oven to 180c (Fan oven)
In a mixer beat the butter until softened
Add the sugars and beat until pale and fluffy
Add the egg, egg yolk and vanilla then mix
Add the flour, cornflour, baking powder and salt then mix
Add the chocolate chips and sprinkles then mix
Transfer to a lined 8 inch cake tin. Add some more chocolate chips and press them in gently
Bake for 20-25 minutes
Cool on a wire rack
For the Icing
In a microwaveable bowl, microwave the chocolate for 1 minute, stir, then 30 seconds, stir, then 10 second intervals stirring in between until melted. Leave to cool
In a mixer whisk the butter until soft
Add the icing sugar and whisk until light a fluffy
Add the cooled chocolate and whisk until combined
Transfer to a piping bag with a star nozzle. Pipe around the edge of the cookie.
Add sprinkles.
