Preheat the oven to 180C/350F/gas mark 4. Grease a 23cm/9inch square cake tin and line with baking parchment.
Mix the ground flax seed and water in a small bowl and set aside to go gloopy.
Sift together the flour, pumpkin spice, bicarbonate of soda and salt. Set aside.
Place the vegan butter and sugars in a bowl and whisk with an electric mixer until smooth and fluffy.
Add the pumpkin puree, vanilla extract and flax seed mixture and whisk until smooth. It will look a bit split and gross - don't worry!
Add the dry ingredients and fold in until well combined then stir in the milk followed by most of the chocolate chips; reserving a handful.
Spread the batter out evenly in the prepared tin and scatter over the reserved chocolate chips, press them into the batter slightly.
Bake for about 30 minutes until a skewer inserted into the centre comes out clean or with a few moist crumbs.
Leave to cool in the tin then flip the cake out and slice into squares. Store in an airtight container for up to five days.