Method...

1. 1. Making the Cookies:
2. 2. In a medium bowl, whisk together the flour, cornflour, bicarbonate of soda, and salt. Set aside.
3. 3. Using an electric mixer on medium speed, cream together the butter, brown sugar, and granulated sugar until light and fluffy. Add the eggs and vanilla, mixing until combined.
4. 4. Gradually add the flour mixture, mixing just until incorporated. Fold in the chocolate chips.
5. 5. Cover and chill the cookie dough for 1 hour.
6. 6. Preheat the oven to 180°C. Line baking sheets with parchment paper or silicone liners.
7. 7. Baking the Cookies:
8. 8. Lightly flour your work surface and roll out the dough until it’s 1/2 inch thick. Use a cookie cutter to cut out shapes.
9. 9. Bake one tray at a time for 9-11 minutes, or until the edges have started to brown. (Do not over-bake for soft, chewy cookies.)
10. 10. Allow the cookies to cool on the baking sheets for 5 minutes, then transfer them to wire racks to cool completely.
11. 11. Making the Filling:
12. 12. Using an electric mixer with a whisk attachment, beat the butter on medium speed until light and fluffy.
13. 13. Reduce the speed to low and gradually add the icing sugar, mixing until smooth and fluffy.
14. 14. Spread or pipe the filling onto the bottoms of half of the cookies. Sandwich with the other half and enjoy!