Preheat the oven to 180C/350F/gas mark 4. Grease and line three 15cm/6in round cake tins (or two 20cm/8in ones).
In a large bowl, whisk together the pumpkin, oil, milk, sugars and vinegar.
In a separate bowl, sift together the flour, baking powder, bicarbonate of soda, spices and salt.
Pour the wet ingredients into the dry (or tip the dry into the wet, it doesn't matter which way round) and fold gently until well combined. Be careful not to over-mix the batter however.
Divide the batter between the prepared tins and spread level. Bake for around 25-30 minutes until well risen and a skewer inserted into the centre comes out clean.
Leave the cakes to cool in the tins for 15 minutes then carefully turn out onto a wire rack to cool completely.
Turn the oven down to 160C/320F/gas mark 3. Line a baking tray with baking parchment.
In a bowl, toss together the pecans, cinnamon, salt, sugar and maple syrup. Make sure that the pecans are well coated.
Spread the pecans out in a single layer on the prepared baking sheet. Bake for 15 minutes then remove from the oven, give them a good stir and return to the oven for 5-10 minutes, until they feel dry to the touch.
Leave to cool on the tray then transfer to an airtight container.
Whisk in the ground cinnamon to taste.
Trim the tops of the cooled cakes to level them. Place one cake layer on a cake stand or serving platter and spread over some of the buttercream.
Place another layer on top and spread with more buttercream. Place the final layer on top upside down (to give a perfectly flat and crumb free surface).
Spread buttercream over the top and sides of the cake, using a bench scraper to get it as smooth as you can. If you are struggling to get it smooth; place the cake in the fridge for 20 minutes to firm up then try again.
Place the remaining buttercream in a piping bag fitted with an open star nozzle and pipe blobs around the edge of the cake.
Stick a ring of pecans around the bottom of the cake.
Roughly chop a couple of handfuls of the pecans and scatter them over the top of the cake inside the ring of buttercream.
Store any leftovers in an airtight container at room temperature for up to a couple of days.
The cakes can be baked in either three 15cm/6in tins (as shown in the images), or in two 20cm/8in ones.