Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Line a 20cm / 8in square cake tin with baking parchment, leaving some overhanging either side so that you can easily lift the bars out later.
Whisk together the pumpkin puree, melted butter, light brown sugar and vanilla extract in a large bowl.
125 g (½ cup) pumpkin puree, 120 g (½ cup) vegan block butter, 175 g (1 cup) light brown soft sugar, 2 teaspoons vanilla extract
Stir in the baking powder, bicarbonate of soda, salt, pumpkin spice and oats, followed by the flour.
1 teaspoon baking powder, ½ teaspoon bicarbonate of soda (baking soda), ¼ teaspoon salt, 1 Tablespoon pumpkin spice, 150 g (1 + ⅔ cup) porridge oats/rolled oats, 120 g (1 cup) plain (all-purpose) flour
Fold in the chocolate chips and nuts.
100 g (⅔ cup) chocolate chips, 100 g (1 cup) roughly chopped pecans or walnuts
Transfer the mixture to the prepared tin and spread it level.
Bake for about 25 minutes, until golden brown and set. They may still be a little soft in the middle but shouldn't jiggle.
Leave the bars to cool completely in the tin. For neat slices, chill the bars in the fridge for a couple of hours once they have cooled before slicing them.