Preheat the oven to 180°C / 160°C Fan / Gas 4. Prepare an 8"/20 cm square baking tin, lining it with baking parchment, and then greasing with butter, cake release spray or a light neutral oil.
Melt the butter and 50g of the peanut butter, either in a saucepan or in a large bowl in the microwave. Don't let it get too warm. You want it to melt but it shouldn't be hot. When the butter is half melted, that's enough. Mix well.
Next, add the sugars. Stir them in well.
Add the egg and vanilla extract. Stir in.
Add the flour, along with any other dry ingredients (baking powder if needed), salt and chocolate chips if wanted . Fold in until you have a glossy batter.
Transfer the batter to the prepared baking pan and give it a gentle shake to level it out.
Melt the remaining peanut butter in a microwave, or in a small saucepan on a low heat. Don't heat it so much it burns or starts to separate: just warm enough to be runny is perfect. 20 seconds in my microwave is perfect for me; your microwave might be more powerful.
Pour the peanut butter over the batter and swirl it in, making sure you get it into the corners. I find the easiest way to do this is with the handle of a teaspoon.
Bake for 30–35 minutes, until a papery crust forms. The edges will have risen and become crinkly. The centre will still be soft, barely set, and will wobble if you (gently) shake the pan.
Allow to stand for 10–15 minutes on a cooling rack. The centre will carry on cooking and firm up as the blondie cools.
Cut the cool peanut butter blondies into squares to serve. I find a 4 x 4 grid is perfect.