For the crust, place the Oreos and salt into the bowl of a food processor and blitz until you have a mixture that resembles breadcrumbs. Drizzle in the slightly cooled brown butter and pulse until the mixture comes together. Pour into a slightly greased 9-inch springform cake pan, and press into the base and sides to form a crust. Place into the fridge for at least 1 hour.
For the filling, finely chop the chocolate. Put the cornstarch into a cup and whisk in the milk until smooth.
Put the cream, finely chopped chocolate, cocoa, sugar, vanilla paste, brown butter and salt into a large saucepan and place over a medium heat, stirring gently until the chocolate has melted.
Remove from heat and whisk in the cornstarch and milk mixture until incorporated then put the pan back on the heat and whisk constantly until the mixture thickens.
Pour the mixture into the crust. Place in the fridge overnight. (Don’t leave it longer than 24 hours, as the base will start to soften.)
When ready to serve, remove the pie and make the meringue. Using an electric mixer on high, beat egg whites until loose and foamy, about 2 minutes. With motor running, gradually add sugar and beat until stiff, shiny peaks form, about 5 minutes. Gradually add powdered sugar and beat another 2 minutes.
Spoon meringue over pie, swirling decoratively. Use a blow torch to toast meringue, if desired.