Preheat the oven to 175°C (350°F). Grease or line an 8-inch round cake tin.
In a large bowl, cream the butter and sugars together until light and fluffy.
Add the eggs and vanilla extract, mixing until smooth.
In a separate bowl, whisk together flour, bicarbonate of soda, and salt.
Combine wet and dry ingredients, mixing until just combined.
Fold in the chocolate chips, then spread the dough evenly into the prepared tin.
Bake for 22-25 minutes until golden at the edges and soft in the middle.
Cool completely in the tin before decorating.
To make the buttercream:
Beat butter until creamy. Gradually add icing sugar, mixing until fluffy.
Add vanilla and milk or cream to reach a smooth consistency.
Tint with green food colouring.
Pipe swirls around the cooled cookie cake and finish with Christmas sprinkles.