For the jam (1 medium-sized jar):
Mix the blackberries with sugar and lemon juice, then allow to macerate for ½ hour.
Then put in a pot over low heat and cook for about 45 minutes or until the mixture has been reduced and caramelized. Once the jam is done, you store it or wait until it cools and then blend to eliminate all clumps.
Grind the peanuts in a heavy-duty food processor at 1 or 2-minute intervals so the appliance does not overheat. Once ground to a paste, the peanut butter is ready to be served and can be stored in glass jars.
To make the sandwich, toast the bread in a grill pan, then fill it with peanut butter, blackberry jam and banana slices.