How to Make It
Place the blackberries in a medium bowl; roughly mash them with a potato masher. Stir in the jam, lemon juice, and ½ cup of the confectioners’ sugar; set aside. Stir together the peanut butter, cream, and remaining ¼ cup confectioners’ sugar in a separate bowl.
Process the peanuts in a food processor until finely chopped. Transfer them to a large bowl.
Heat the oil in a deep fryer until a candy thermometer reads 350°F. (If you don’t have a deep fryer, you can use a Dutch oven.) Separate the biscuit dough. Working in batches, fry the dough, turning once, until deep golden brown and cooked through, about 2 minutes per side. Transfer to a plate lined with paper towels to drain for 1 minute. Roll the hot doughnuts in the crushed peanuts.
Using a chopstick or wooden skewer, poke a hole into the side of each doughnut, wiggling the chopstick from side to side to create a ¼-inch hole. Transfer the peanut butter mixture to a large zip-top plastic freezer bag. Snip one corner off the bag. Pipe the peanut butter mixture into each doughnut until it almost bursts and the mixture begins to ooze from the side of the doughnut. Sprinkle the doughnuts with any remaining crushed peanuts. Serve with the jam mixture for dipping.