Set the butter in a heatproof bowl. Heat it in the microwave (use a microwave plate to stop the butter from sputtering out) until it’s partially melted.

Add the brown sugar, salt and vanilla to the bowl and whisk very well, for about a minute.

Return the bowl to the microwave and heat for 1 minute. Whisk the mixture very well, it will start as loose and separated and as you whisk turn to a thick and shiny mixture - don’t stop whisking until you see this transition.

Add the egg yolks and egg and whisk well. Add the sourdough discard and whisk until it’s well combined.

Stir in the flour. The batter will be warm and loose, cover the bowl with plastic wrap to seal and set it in the fridge for 45-60 minutes. Or, leave it overnight if you prefer.

Preheat the oven to 350 F. Grease then line an 8 or 9 inch square pan with parchment paper, use metal clips to hold the paper in place.

Stir the chocolate into the batter (we add it when the batter is cool so the chocolate doesn’t melt) and spread into the prepared pan. Nudge the batter into the corners of the pan and smooth it into an even layer.

Bake the blondies until the edges are golden and the center has puffed, about 30 minutes. If you’ve done the heating and whisking correctly, you’ll see a lovely shiny top formed on top.

Out of the oven the blondies will deflate, once cool you can remove them from the pan to slice and serve.