Preheat oven to 170°C and butter a large (25 x 45cm) Swiss/jelly roll tin.
Place butter, sugar, maple syrup, salt and vanilla into TM bowl and blend 30 sec/speed 6. Insert Butterfly. Whip 1 min/speed 4. Remove Butterfly.
Add eggs and baking powder and blend 30 sec/speed 8. Scrape down sides of bowl.
Add the flour and blend 10 sec/speed 8.
Add chocolate and nuts and blend 5 sec/speed 8. Use your spat-ooh-lah-lah to combine the rest.
Spread into prepared pan and bake 15 minutes. Depending on your oven, add a further 5 minutes. Until golden and just set in the centre. You are going for slightly undercooked. They will continue to cook out of the oven.
Remove and cool. They will improve on standing. I found they were better the next day.
NOW to the chewy part...I have tried to reduce the sugar in this bar slightly. But you will find that the more sugar you add the chewier the bars become. So the lowest level I recommend is the 350g mark, the upper mark being 450g. Experiment. I find the 350g chewy enough the next day. But not straight out of the oven. Also, using a dark pan vs a light coloured pan will result in different cooking times. Darker the pan, faster they will cook.