Heat the oven to 180°C/160°C fan/350°F/ Gas 4. Sift the cocoa powder, flour and baking powder into a mixing bowl, then stir in the salt and sugar.
Gently whisk the oil and warm coffee or water in a measuring jug, then whisk in the vinegar.
Pour the liquid mixture into the bowl with the dry ingredients and stir well until thoroughly combined. Add the chocolate chips or chopped chocolate, and stir until evenly distributed.
Transfer the mixture to the prepared tin, spreading it evenly, and making sure the corners are properly filled. Scatter the nuts over the top, then bake for 25 minutes, or until a skewer inserted at a mid-point between the sides and the centre of the brownie comes out just clean (you’re aiming for a squidgy brownie, rather than a dry one).
Remove from the oven, place the tin on a wire rack and leave the brownie to cool completely before cutting it into 16 squares in the tin. Dust with icing sugar before serving, if you wish.
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