Preheat the oven to 180C (gas mark 6). Line a loaf tin with baking paper.
In a large bowl, mix together the plain flour, corn flour, sugar, cocoa powder and salt. Mix well until there are no lumps.
Pour in the soya milk, melted chocolate, vanilla extract and oil, and mix again until a thick batter forms.
Next, mix through the pistachios and chocolate pieces and spoon the mixture into the lined baking tray.
Cook for 20 minutes. Reduce the oven temperature to 170C (gas mark 5) and cook for further 10-15 minutes, until the middleof the cake has set and a toothpick inserted in the center comes out clean.
Let cool slightly, 15 to 20 minutes. Meanwhile, for the tahini icing, place the tahini, syrup, coconut oil and cinnamon in a bowl and whisk until smooth. Drizzle brownie with tahini icing.