Line a 10-in/25-cm cardboard cake round with a parchment circle of the same size. Cut the Joconde sheet into three 8-in/20-cm squares.
Combine the chocolate and vegetable oil. Turn one Joconde layer upside down and thinly coat the top surface with the chocolate mixture. Place it chocolate side down on the parchment-lined cake round. Moisten the layer with simpl