Preheat the oven to 160°C/Gas 3 and grease the tin with cake release spray.
Melt the chocolate squares and butter in a saucepan over a medium heat.
Remove from the heat and stir in the sugar until well mixed.
Add the eggs one at a time and mix well. Add the vanilla, nuts and chocolate chunks. Stir in the liquid glucose, add the flour and mix thoroughly.
Pour the mixture into the tin and bake for 50 minutes in the centre of the oven. A skewer should come out slightly sticky, not clean, otherwise the brownies will be dry and overcooked rather than fudge-like.
Remove from the oven and leave the tin to cool on a wire rack, then follow the portion guides to cut into 20 identical-sized pieces.