Heat the oven to 180C/170C fan/gas 4. Butter a roughly 20cm square baking tin and line with baking parchment. Melt the peanut butter, butter, brown sugar and vanilla together in a large saucepan over a low heat, stirring occasionally until combined. Remove from the heat and leave to cool slightly.

Whisk in the eggs and a generous pinch of salt, then sift over the flour and baking powder, and fold in using a spatula until just combined. Pour the mixture into the prepared tin. Dot the jam over the top, then use a skewer to gently swirl it in and create a marbled effect.

Bake for 30-40 mins, or until the blondies are shiny on top with a gentle wobble in the middle. Leave to cool completely in the tin. If you like, chill the cooled blondies for 1 hr if you prefer them to be fudgy. Slice into 16 squares.