Sourdough brownies with a tahini swirl recipe

Ingredients

• 150 g dark chocolate (at least 70% cocoa solids), broken up
• 50 g unsalted butter
• 60 ml rapeseed or sunflower oil
• 2 eggs and 1 egg white
• 1 pinch of fine salt
• 100 g caster sugar
• 80 g light brown sugar
• 3 tbsp water (at least)
• 1 tsp vanilla bean paste
• 50 g unsweetened cocoa powder
• 120 g sourdough starter (made with 50:50 water:flour)
• 75 g walnuts, chopped (optional, omit if nut-free)
• 2 tbsp tahini
• 2 tsp maple syrup, honey or agave