First, make the base. Blitz up the Oreos in a food processor or bash with a rolling pin until fine crumbs. Add the melted butter and stir together. Press into a 24cm tart tin (loose bottomed ones are great). Set aside.
Make the ganache: chop up all the chocolate really small. Put in a large heatproof bowl.
In a small saucepan, heat the cream and butter on a low heat. Do this until the butter has melted and the cream is beginning to bubble around the edges. Immediately pour over the chocolate and let sit two minutes.
Stir the ganache/chocolate/cream mixture together with a spatula. It might look odd to begin with but within seconds watch it transform to a glossy chocolate!
Sprinkle the raspberries over the tart base. Pour over the ganache and spread evenly. Chill in the fridge for an hour (this can now sit overnight or until ready to serve).
Whip cream using a handheld or manual whisk until soft peaks are forming. Spread over the tart, add extra raspberries and chocolate shavings (do this by simply grating chocolate on a fine cheese grater). Serve!