• Place the potatoes in a baking dish and bake in the oven for 45–60 minutes, depending on their size, until tender (a knife should insert easily through the potato when it is cooked). Remove the potatoes from the oven and leave to cool before peeling and mashing the sweet potato flesh.
• Place the sweet potato flesh in a large bowl and add the sugar and oil. Use an electric hand-held whisk to beat until smooth. Add the eggs a little at a time, beating well after each addition.
• In a separate bowl, sift together the flour, raising agents, salt and spices. Stir into the wet ingredients and mix until just combined but no lumps remain. Stir in the vanilla.

For cupcakes:

• Spoon the batter into 12 hole muffin tins lined with paper cases and bake for 20-25 minutes until springy on top and a skewer inserted into the centre of the cakes comes out clean. Allow the cakes to cool slightly in the tin for a few minutes then transfer to a wire rack to cool completely.
• If making a large cake:
• Transfer the cake batter to the prepared tin and bake for 60–65 minutes or until a skewer inserted in the centre comes out clean. Transfer the cake to a wire cooling rack. Run a small knife around the edge of the cake tin and leave to cool for 15 minutes before turning our and leaving to cool completely. Transfer the cooled cake to a serving plate or cake stand.

For the icing:

• Stir 4 tablespoons of caramel sauce into the buttercream in a bowl. For cupcakes, streak another tablespoon of caramel sauce down the side of a large disposable piping bag and fill the bag two thirds full with the caramel buttercream. Pipe spiral swirls onto the cooled cupcakes and grate dark chocolate over the tops and serve. If making the large cake, spread the toffee buttercream over the top and sides of the cake with a palette knife. Decorate around the base of the cake with the crystallised ginger and drizzle the remaining toffee sauce over the cake.