Lightly grease 4 individual fluted tart tins. The tins should have a removable bottom.

In a food processor crush the Oreos. Alternatively, seal the Oreos in a freezer bag and bash them with a rolling pin, until they are finely crushed.

Add 120g of unsalted butter to a small heatproof bowl, and place over a small saucepan of simmering water. Make sure that the bowl does not touch the water. Heat until all the butter has melted. Alternatively melt the butter in the microwave.

In a large bowl, add the melted butter to the crushed Oreos, and mix together until well combined.

Add the Oreo mixture into each of the tart tins, and press the mixture into the bottom and up the size of the tin. Make sure the mixture is firm, and the sides are high enough to hold the filling.

Spoon the caramel sauce into each of the tins, and smooth over, making sure the bottom is completely covered.

In a medium saucepan, add the double cream, and gently heat until the cream comes to the boil.

In a mixing bowl, add the chocolate and 60g unsalted butter. Pour the cream over the chocolate and butter and let the mixture rest. After 5 minutes, gently whisk the mixture until the chocolate and butter has melted, and the mixture is smooth. Carefully, pour the chocolate ganache into the tins, over the caramel layer.

Chill the tarts in the fridge for an hour, and dust with cocoa powder before serving.