Place the cookies, including their filling, in a food processor and pulse until you have fine, even crumbs. Transfer the crumbs to a medium bowl and stir in the melted butter until everything is evenly moistened.

Press the mixture firmly over the bottom and up the sides of a 9-inch (23 cm) tart pan, creating an even crust. Place it in the fridge for about 30 minutes to firm up before filling.

For a crisper crust, I highly recommend baking it instead. Bake the crust in a preheated 350°F (180°C) oven for 5 to 6 minutes, then transfer it to a wire rack and allow it to cool completely before adding the filling. This extra step gives the crust a firmer, pleasantly crisp texture rather than leaving it too soft.

Place the chopped bittersweet chocolate and milk chocolate in a medium heatproof bowl and set aside.

In a small saucepan over medium heat, bring the heavy cream and butter to a gentle boil. Remove the saucepan from the heat and immediately pour the hot mixture over the chopped chocolate.

Let everything sit undisturbed for 1 minute. Then use a rubber spatula to slowly stir until the chocolate has completely melted and the ganache is smooth and glossy. If a few pieces of chocolate remain, warm the mixture in the microwave for several seconds at a time, stirring between each interval, until perfectly smooth.

Pour the chocolate ganache over the chilled Oreo crust and place the tart in the fridge until the filling is completely set, at least 4 hours or preferably overnight.

This tart can easily be prepared a day ahead, making it a lovely dessert to have ready when guests arrive.

Once covered well, the tart will keep in the fridge for 4 to 5 days.