Preheat the oven to 170°C/150°C Fan/325°F. Put the butter, sugar and maple or golden syrup into a small heavy-based saucepan and gently warm, stirring once or twice, until the butter's melted and the sugar has dissolved. Remove the pan from the heat.Fork together the flour, cocoa and salt to mix, then beat into the butter and sugar pan with a wooden spoon or spatula until smoothly combined.Whisk the egg with the vanilla - just casually, by hand - then stir into the pan, giving a final little whisk, if needed, to make sure everything's mixed together thoroughly, before folding in the nuts and chocolate chips. Pour and scrape this nubbly brownie batter into a foil tin measuring approx. 18 x 11 x 5cm / 7 x 5 x 2in. Bake in the oven for 15-20 minutes until it is beginning to come away at the sides and the top has dried a little just around the edges.Transfer to a wire rack and leave to cool - but not completely. I leave them for 20-30 minutes (and that's difficult enough) so that my first bite gets them when still warm, but just set enough to cut into 2 squidgy squares. If your need is not so great, or you want them to go further, cut each square in half again.