Grease a 8-9" quiche/tart tin with butter and leave to the side
Using a stand mixer (or a wooden spoon and elbow grease) cream together the unsalted butter, caster sugar and light brown sugar until soft and creamy - can take up to a minute!
Mix together the medium egg and vanilla extract and beat slowly into the sugar mix until all combined well
Add in the plain flour, bicarbonate of soda and salt and mix again until combined - it shouldn't take too long at all!
Once combined, add in the chopped milk chocolate and fold through till even
Press down the cookie dough with your hands into the tart tin and bake in the oven for 16-18 minutes until golden on top, and cooked through
Leave to cool in the tin completely!
Once cooled completely, using a separate bowl and an electric whisk, whisk together the double cream, mascarpone, icing sugar, and vanilla extract for 2-3 minutes until thickened considerably and mousse like - its not essential for it to hold itself like a meringue, just to be thick!
Remove the giant cookie from the tin, and fill with the cream mixture
Add in the Oreos and sprinkle with crushed cookies and more chocolatey bits
ENJOY!
Notes
This dessert should be kept covered, and in the fridge for up to 3 days!
This delicious dessert is best on the day its served, and if you find the cookie is quite hard around the edges once baked, remove from the fridge about 10 minutes before serving so its not quite so cold and easier to cut! (If the cookie is baked correctly, this won't be a problem!)
You can always use dark/white chocolate, but milk is my all time favourite!