Preheat the oven to 350 degrees F, and grease 6 (8-ounce/half-pint) canning jars. In a medium size bowl, combine the flour, cocoa powder, sugar, baking powder, baking soda and salt. In a separate large bowl, thoroughly combine the espresso mixture, eggs, milk, and sweetened condensed milk. Add the dry mixture to the wet mixture and stir to fully incorporate.
Evenly fill the jars with the batter. It will fill the jars about 3/4 of the way. Sprinkle the chips evenly over the top.
Place the jars in a deep casserole or baking dish and then add water to the dish so that it reaches about 1/3 of the way up the jars. For added ease in transferring to and from the oven, you can place the dish on a baking sheet with sides. Bake approximately 25 minutes, give or take a few depending on oven, or until the cakes pass the toothpick test. (Like a brownie, I have found that a hint of undercooking is enjoyed by many. But a word of caution: the batter of the rice flour version has a hint of graininess, and this may be detected if cakes made with rice flour are undercooked. They still taste great, though!)
Once the jars are removed from the oven, place them on a wire rack to cool. Store any uneaten cakes, covered, in the refrigerator where they will maintain freshness for at least 3 days. The cakes can be removed from the jars by running a butter knife around the edges and gently prying them out. However, we like to eat them right from the jar!
