Preheat the oven to 190 C / Gas 5. Grease and line a 23x33cm Swiss roll tin with baking parchment.
Prepare the filling by whisking together the pumpkin purée, double cream and cinnamon. Set aside.
In a mixing bowl, whisk the eggs and sugar until pale and fluffy. Add the water, salt, flour and baking powder and continue whisking. Spoon into the prepared Swiss roll tin and gently smooth the top.
Bake in the preheated oven for 10 to 15 minutes until the sponge is springy and just pale golden brown, any longer and you risk overcooking the sponge and it cracking during rolling.
While the sponge is in the oven prepare a sheet of parchment by laying it out on a hard surface and sprinkling with caster sugar (all the way to the edges).
Remove the sponge from the oven and allow to cool for 5 to 10 minutes. In one swift motion, flip the tin over onto the prepared sheet of parchment and gently peel off the backing layer of parchment, and discard.
Very carefully, score a line across the bottom of the sponge 2cm in width at the end nearest to you; this will be the very centre of the Swiss roll when rolled up. Cover the sponge with a second piece of parchment, then roll the sponge up tightly starting at the end which has been scored. Allow to cool fully, then then gently unroll and discard the inner sheet of parchment.
Spoon the filling onto the Swiss roll, leaving a gap of 2cm away from the edge. Gently roll up the Swiss roll and place the seam side down on a serving plate. Dust with icing sugar and serve.
Homemade or shop bought pumpkin purée both work great for this recipe. Follow this recipe to make homemade.