1) Position a rack in the centre of the oven and preheat to 180C/Gas 4. Lightly grease, then line a 33 by 45cm jelly-roll pan with parchment or waxed paper. (Let the paper come up the long sides by about 5cm - this will make it easier to form the roll.) Brush paper with oil or butter, dust with flour, tap out excess.
1) For the cake: Whisk together plain flour, bicarbonate of soda, baking powder, cinnamon, cloves, salt, ginger and cardamom together in a medium bowl. In another bowl, combine sweet potato, buttermilk, and vanilla until smooth.
3) Beat butter and sugar in the bowl of an electric mixer on high speed until light and fluffy, about 4 mins. Reduce to low; add eggs 1 at a time, beating well after each addition. Add flour mixture in 3 parts, alternating with the wet mixture in 2 parts, beginning and ending with flour. Beat until just blended. (If needed, fold batter together with a rubber spatula.)
4) Spread batter evenly in prepared pan. Bake until cake springs back when pressed lightly, about 15 mins.
5) Run knife around cake edge, and invert cake onto a cotton tea towel (100 percent cotton works best and keeps the cake from sticking) generously dusted with icing sugar. Carefully pull back parchment paper to release, and then place it back on top of the cake. Gently roll up the cake and towel together, starting from the narrow end to make a fat roll.
6) Meanwhile for the filling, whip cream and icing sugar to soft peaks. Fold in crystallised ginger.
7) Carefully unroll the cake and remove the towel. Drizzle caramel over the cake. Spread gingered cream evenly over the cake. Using the short edge of parchment as guide, roll the cake up into a fat, loose cylinder. Transfer to a long platter. Dust with more icing sugar and cinnamon in a Halloween pattern if desired. Serve, now or refrigerate until ready to serve.