If starting with fresh pumpkin (to make 200 g purée): Heat oven to 190°C (170°C fan). Peel, deseed, and chop about 400 g pumpkin; roast on a lined tray for 25–30 minutes until tender. Cool slightly, then mash or blend until smooth; measure 200 g for the recipe.
Keep the oven at 190°C (170°C fan). Line a 22 × 12 cm loaf tin with baking paper.
Rub the butter into 30 g of the flour until crumbly. Stir in 25 g of the sugar along with the walnuts and half a teaspoon of cinnamon, then chill until needed.
In a large bowl, whisk the remaining flour, baking powder, bicarb, cinnamon, ginger, nutmeg, and salt.
In another bowl, whisk the pumpkin purée, maple syrup, brown sugar, eggs, oil, and vanilla until smooth.
Fold the wet mixture into the dry ingredients until no dry patches remain. The batter should be thick but scoopable; add 1–2 tbsp milk only if very stiff.
Spoon the batter into the prepared tin and level the top. Scatter the walnut crumble evenly over the surface.
Bake for 55–65 minutes, until deep golden and a skewer inserted into the centre comes out clean.
Brush the top lightly with maple syrup or pumpkin spice syrup (optional). Cool in the tin for 10 minutes, then lift out to a rack to cool completely.