Cream butter & sugar in the bowl of a stand mixer on high speed until light & fluffy.
Add the egg and vanilla and beat until well combined.
Add flour, cocoa powder, cornflour, baking powder & salt and mix in well.
Split dough in half and shape each half into a roll about 30cm long, try to make it as perfectly round as possible. Wrap in plastic wrap and chill in the freezer for 1 hour, fridge for at least 4 hours and even overnight. At this point, you can also put the rolls in the freezer to use at a later date.
Pre-heat the oven to 180 degrees C.
Remove the cookie roll from the fridge or freezer and unwrap. If using frozen dough, allow to thaw in the fridge for an hour or so.
Using a sharp knife, carefully slice into 1/2 cm thick slices, trying not to squish the rolls. Lay out the cookies on a lined baking tray and bake for 12-14 minutes, checking them at 12 minutes. You want to bake them for as long as possible before they start to burn, in order for them to be as crunchy as possible. 14 minutes was perfect for me.
Allow to cool on the tray for 5 minutes, then transfer to a cooling rack. Allow to cool completely before filling them with frosting.
To make the frosting, using either a mixer or by hand, combine butter, icing sugar and milk until it is a spreadable consistency, adding more milk or icing sugar as necessary. Add the ingredients required to make the flavour of frosting you want. You can even split the plain frosting in 3 and add smaller amounts of each flavouring to have all 3.
Spread a good amount of frosting on 1 cookie and sandwich another on top. Repeat with all cookies.
Cookies will keep in an airtight container for 1 week but will start to soften over time. For maximum Oreo crunch, eat within 2 days