1. Preheat oven to 180C. Line 2 baking sheets with baking paper.
  2. Whisk together flour, baking powder, baking soda, and salt.
  3. Beat peanut butter and butter in a stand mixer on medium speed until smooth. Add sugars, and beat until pale and fluffy. Add egg and vanilla, and beat until incorporated. Reduce speed to low. Add dry ingredients, and mix until combined.
  4. Scoop level tablespoons of dough, and form into balls. Roll each ball in extra caster sugar, and transfer to lined baking sheets, spacing about 2 inches apart.
  5. Bake until cookies are puffy, about 10 minutes. Remove from oven, and make indentations in centers by pressing with the handle end of a wooden spoon. Spoon ½ tsp of the jam into each indentation. Return to oven, and bake until edges are golden, 6 to 7 minutes. Transfer sheets to wire racks, and let cool completely.

For extra decadence, try using these cookies to make ice cream sandwiches!