To make Dulce de Leche: remove paper labels from the can of condensed milk. Fill a large pot with water, enough to cover the can. Let the water boil, then simmer over medium heat to cook the condensed milk inside the can. Cook for 3 hours. If using a Slow Cooker like I did, cook the can of condensed milk on HIGH settings for 4 hours, with water covering the can. After cooking time, leave the can submerged in the water and let it get to room temperature. Then refrigerate can of caramel condensed milk overnight and set aside till ready to spread as cookie filling.
To make the chocolate cookies:
Preheat the oven to 350 F degrees.
Line baking sheets with parchment paper.
In a large bowl, place all the ingredients together : flour, sugar, cocoa powder, baking powder, salt, eggs, corn oil, vanilla and rum. Using a stand mixer, mix all the ingredients till well blended. Batter will get thick and easier to handle by hand.
Shape the cookie dough into round chocolate balls about 2 inches in diameter. Arrange the cookie dough balls on the cookie sheet, about 2 inches apart.
Bake at 350 F degrees for 9 to 10 minutes. When cookies are done, cool on a rack for at least 30 minutes.
Make sure cookies are thoroughly cooled before placing the filling.
To assemble chocolate cookies : Take the bottom cookie, spread a tablespoon of the caramel dulce de leche (from the cooked condensed milk). Place another cookie on top of the filling to form a chocolate sandwich. Press cookies together. Wrap in plastic and freeze cookies till ready to serve.
Cook's comments: To make this cookie in stages, mix the cookie dough and keep in an air tight covered plastic container. Freeze at least overnight then bake the following day. The frozen dough keeps in the freezer for 2 to 3 weeks. The dulce de leche is better if cooked the day before.