In a large saucepan, bring double cream and milk to a simmer over medium heat. When it begins to bubble, turn off the heat and cover with a lid. Steep for 30 minutes and then let it cool to room temperature.
Add mint and sugar into a food processor and grind until it looks like pesto. Set aside.
Whisk sugar + mint, egg yolks and salt in a medium bowl. Return milk and cream to a simmer. Using a ladle, transfer ½ cup of the hot dairy mixture into the yolks. Whisk to combine, then repeat with a second and third ladleful. Pour warm eggs into the pot and cook over medium/low heat, stirring with a heat proof spatula until it begins to steam. Strain through a fine mesh sieve. When you do this, it will remove the extra mint leaves that are leftover. Stir in vanilla extract. Cool to room temperature and then refrigerate for 4 hours.
Pour mixture into your ice cream maker and churn according to your machine's directions, until fluffy and light. Add in the chopped chocolate. Freeze the ice cream so it gets a bit firmer, but you still want it to be soft in order to put it into the cake pan.
In a food processor, grind the Oreos with the melted butter until you get a damp sand mixture.
Line an 8 inch cake pan with clingfilm. Scoop the ice cream out into the cake pan. Once all the ice cream has been put in, pat it down with a glass or metal cup to make it smooth. Then add the oreo cookie base on top, smoothing it down. Put it back in the fridge for 12 hours.
Take the pan out of the freezer and place it cookie side down on to a plate, and lift the pan. If your ice cream was super cold it might take a minute or two, but with the clingfilm it should come away easily!