Cut out two pieces of parchment paper, each about 15x10-inches. Set aside.

In a small bowl, mix together the raspberry jam and cornflour until smooth. Set aside.

In a large bowl, beat together butter, peanut butter, and sugar until light and fluffy. Add the egg and vanilla, and beat until combined.

Add the flour, baking powder, and salt, and mix until a thick dough forms. Use your hands to bring the dough and any crumbs together.

Place the dough onto one of the prepared pieces of parchment paper. Press the dough out slightly using your hands, then top with the second piece of paper.

Gently roll the dough out enough to reach the edges of the paper. Remove the top sheet, then spread the jam evenly over the dough, leaving about 1cm around the edges.

Carefully loosen the edge of the dough on the short side of the paper. Roll the dough up tightly like a Swiss roll, making sure to seal the seam and keeping it on the underside of the roll.

Wrap the roll in clingfilm, and freeze for 1 hour or until firm.

Preheat the oven to 180C/350F/Gas 4, and line a baking tray with parchment paper.

Slice the roll into 1/2-inch thick slices, and place them onto the prepared baking tray, about 2-inches apart. Bake for 18 - 20 minutes or until set and slightly golden brown around the edges.

Allow to cool for 5 - 10 minutes before transferring to a wire rack to cool completely.