Pre-heat oven (180 °C, Fan 160 °C, 350 °F, Gas 4), grease and line a 20x20cm (8x8 inch) square baking tin with baking paper.
In a large bowl sift together the dry ingredients (flour, sugar, cocoa powder, baking powder, salt), then stir in liquids (soy milk, melted margarine, treacle) until a smooth batter forms. Finally, mix in walnut pieces.
Pour batter in to the lined cake tin, smoothing it out and distributing the walnuts evenly.
Bake in the centre of the oven for 30-35 minutes. Towards the end of this time test by inserting a cocktail stick into the middle of the cake. It should come out clean. If there is wet batter on the stick, return it to the oven and cook for a couple of minutes more, then test again. Repeat as needed until the stick comes out clean.After 10 minutes remove the cake from its tin and leave to cool completely on a wire rack before slicing.
Notes
These slice best after cooling...but they're also awesome hot and fresh from the oven. I'll let you figure out this dilemma your way : )