Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Grease a deep, loose-bottomed 20 cm/8 in round cake tin and line the base and sides with baking parchment.
Start by preparing the crumble topping - mix the plain flour, light brown soft sugar and pumpkin spice together in a bowl. Add the cold, diced vegan block butter and rub it in using your fingertips until the mixture resembles breadcrumbs, is clumping together and no lumps of butter remain. Stir in the chopped pecans then pop the bowl in the fridge while you prepare the cake batter.
To make the cinnamon sugar, stir together the sugar, cinnamon and cocoa powder in a bowl. Set aside.
To make the cake batter, whisk together the pumpkin puree, oil, milk, brown and caster sugars, vinegar and vanilla in a large bowl.
Whisk in the pumpkin spice, baking powder, bicarbonate of soda and salt until well combined then whisk in the flour. Mix until no dry lumps remain but be careful not to overbeat the batter.
Spread half of the batter across the base of the prepared tin. Scatter over the cinnamon sugar evenly.
Blob the rest of the cake batter on top and gently spread it out so that the cinnamon sugar is fully covered.
Scatter over the streusel evenly and extremely gently pat it down just slightly; don't press it into the batter.
Bake for 60-65 minutes until the cake is firm to the touch and a skewer inserted into the centre comes out clean.
Leave the cake to cool in the tin for 20 minutes, then carefully turn it out onto a wire rack and leave to cool completely before slicing.