Lightly coat the bottom of an 11x7-inch baking dish with vegetable oil and set it aside. Melt half of the chocolate in the top of a double boiler over boiling water. Remove the pan from the heat, add the remaining chocolate, and stir until melted. Remove the top of the double boiler from the bottom and continue to stir for at least 10 minutes more, until chocolate thickens and cools to room temperature. Pour the chocoalte into the prepared baking dish and spread it evenly with a rubber spatula. Let sit for 10 mintues. Cut the chocolate in half crosswise with a sharp knife. For fluted borders, first run a pizza cutting wheel around the entire outer border of the chocolate, then cut in half. Transfer the dish to the refrigerator and chill for about 30 minutes, until the chocolate has hardened. Remove the baking dish from the refrigerator. Holding both Chocolate Cards intact, turn the dish upside down and gently dislodge onto a counter. Decorate and inscribe your message on each card with the cake decorating gel (use white gel on a bittersweet Chocolate Card, and brown gel on a white Chocolate Card). Refrigerate the cards for an additional 10 minutes after decorating.