Preheat the oven to 180° degrees (356 F).
Prepare a water bath (bain marie) by placing a heatproof bowl over simmering water. Combine oil, water, sugar, and chocolate in the bowl. Heat the mixture gently, stirring continuously until the chocolate is completely melted and the mixture turns smooth and glossy. Set it aside for now.
Take a pie tin and carefully line it with the puff pastry, ensuring a snug fit. Set the pie tin aside, and let's move on to the next step.
In a large mixing bowl, combine the remaining ingredients with the prepared chocolate syrup. Mix everything together until all the ingredients are thoroughly incorporated. Pour it into the prepared pie tin, spreading it evenly.
Place the pie tin in the preheated oven and bake for approximately 20 minutes, or until the pastry turns golden brown. After baking, resist the temptation to dig in immediately! Allow the dessert to rest for a few hours.
Once the brownie pie has fully set, add whipped cream on top, followed by some grated chocolate, carefully slice into it and indulge in the rich, chocolatey goodness.