You will need two 1-l/1.05-qt heavy cream whippers and 4 CO2 charges; alternatively, make half the recipe at a time, because 1-l/1.05-qt whippers cannot hold the entire recipe.
Prepare a 25-cm/10-in square frame using 2.5-cm/1-in-thick caramel bars (heavy stainless-steel bars).
Melt the chocolate with the oil over a hot water bath. Divide the mixture equally between the two whipper canisters.
Charge each canister with 2 cartridges, shaking vigorously after each charge.
Immediately after, extrude the chocolate into the prepared frame. Both canisters should fill the frame and there should be excess; pour the excess into a sheet pan lined with a nonstick rubber mat and let it set in the refrigerator. Cut this piece into 4 smaller pieces about 5cm/2in long; they will be used to garnish the cake.
Let set at room temperature. Once set, cut the chocolate into four 10-cm/5-in squares.
Reserve in the refrigerator until ready to assemble the cake.