Preheat the oven to 180C (160C fan forced) / 350F. Line an 8 inch square baking pan with baking paper.

Cook stirring regularly so it doesn’t catch on the bottom. The blueberries will release a lot of liquid which will boil and eventually thicken until you can drag a line through it with the spatula that takes a couple of seconds to fill again about 15-20 minutes after starting. You want the liquid to be very thick - almost as thick as regular jam. It will thicken more on cooling.

Remove from heat and set aside to cool.

Use the large side of a box grater to grate in the remaining cold butter, then gently work it into the flour mixture with your fingertips or a pastry blender until it looks a bit like breadcrumbs and is well distributed. You don’t need the butter to be completely worked in, just in very small pieces.

Stir in the melted butter with a spoon and your hands until well distributed and you can squeeze together a clump in your hand. It will still be a crumbly mixture.

Set aside ⅓ of the mixture for later, and press the remaining ⅔ of the mixture into the base of the baking pan.

Bake the shortbread base for 20 minutes.

Bake for a further 25-30 minutes until just starting to turn golden brown on top.

Cool in the tin for at least 1 hour then you can leave it there or transfer it a wire rack, using the baking paper to lift it out of the pan, to cool completely. It can be served warm but will hold it’s shape better when completely cool.