Preheat the oven to 180C/350F/gas mark 4. Grease a 23cm square cake tin and line with baking parchment.
Mix together the ground flax/linseed/chia and the 3 Tbsp water in a small bowl. Set aside for 10 minutes until gloopy.
Place the vegan margarine, peanut butter, light brown sugar and caster sugar in a large bowl and whisk together until smooth. Whisk in the flax egg and the vanilla extract.
Sift together the flour, bicarbonate of soda, baking powder, salt and cinnamon in a separate bowl. Fold half of the dry ingredients into the butter mixture followed by half of the plant milk then the rest of the dry ingredients followed by the rest of the milk.
Reserve a handful of each of the chocolate chips and pecans then fold the rest into the batter. Pour the batter into the prepared tin and spread level then scatter over the reserved chocolate chips and pecans.
Bake for about 40 minutes until it is just firm to the touch and a skewer inserted into the centre comes out with moist crumbs but no wet batter. Cover loosely with foil about halfway through baking if the top starts to brown too quickly.
Leave to cool in the tin then carefully lift out and slice into squares. Store in an airtight container for up to a couple of days.