Get ready two pieces of baking parchment each about 38x25cm (15x10 in).
In a small bowl, mix the raspberry jam and cornflour together until smooth and the cornflour has dissolved.
In a free-standing electric mixer with a paddle attachment, or using a hand-held electric whisk (or a manual whisk/fork/spoon), cream the butter, peanut butter and the sugar together until light and fluffy. Add the egg and milk. Sift in the flour, bicarbonate of soda and salt, mixing on a slow speed until well incorporated and the mixture forms a dough.
Place the dough on one of the prepared pieces of baking parchment. Press the dough out slightly using your hands, then top with the second sheet of baking parchment. Gently roll the dough out to reach the edges of the paper. Remove the top sheet and spread the jam evenly over the dough, leaving about 1cm (1/2 in) around the edge.
Carefully loosen the edge of the dough on the long side of the paper. Using the edge of the paper as a 'handle', roll the dough up Swiss-roll style, making sure to seal the seam and keeping it on the underside of the roll. Wrap the roll in cling film. Place the roll in the fridge and allow to set for a couple of hours, preferably overnight.
Preheat the oven to 170 C / Gas 3, and line 2 to 3 baking trays with baking parchment.
Once set, slice the roll into 2cm thick slices. Place the slices on the prepared baking trays, about 5cm (2 in) apart.
Bake for approximately 10 to 12 minutes or until golden brown. Remove from the oven and allow to cool completely before serving.