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

Using a handheld or stand mixer fitted with the paddle attachment, beat the butter, peanut butter, and sugars until light and fluffy. Add the egg yolk, and vanilla, and mix until combined.

Add the flour, and salt, and mix to form a smooth, soft dough.

Roll the dough into 0.7oz balls. Roll each one in granulated sugar, and place on the prepared baking tray. Use your thumb, or the back of a measuring spoon (I used a 1/2 teaspoon measuring spoon), to make an indent in the middle of each ball.

Bake for 10 - 12 minutes until firm, and lightly golden brown around the edges. If the indents have risen, gently press them back down with the measuring spoon. Allow to cool for 5 minutes before transferring them to a wire rack to cool completely.

Add the jam to a microwave-safe bowl, and heat until runny. Spoon the jam into each cookie indentation. Allow the jam to cool/set before serving.