In a saucepan off the heat, combine the brown sugar, cocoa powder and cornstarch. Add the remaining ingredients. Bring to a boil, stirring with a whisk, and let simmer for 10 seconds. Pour the sauce into the jars, about 6 tbsp (90 ml) per jar.
Line the bottom of the slow cooker with a clean cloth to prevent the jars from rattling.
In a bowl, combine the flour, baking powder and salt. Set aside.
In another bowl, cream the butter, sugar and cocoa powder with an electric mixer. Add the egg and beat until smooth. At low speed, add the dry ingredients alternately with the milk. With a 1/4-cup (60 ml) ice cream scoop, spoon the batter over the sauce.
Place the jars in the slow cooker. Pour cold water into the cooker to reach halfway up the jars. Cover and cook on High for 2 hours or until a toothpick inserted into the centre of the cake comes out clean. It can be maintained on Warm for up to 2 hours. Remove the jars from the cooker and let cool for 15 minutes.
The chocolate pudding cake will keep for 2 days at room temperature.
If the pudding cake has cooled, place it in the microwave oven for about 30 seconds to reheat the sauce before serving. It will be even better.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.