• The preparation of this terrific cake is straightforward: so breathe.
• Preheat the oven to 180C (356F) degrees.
• Wash the potatoes with their skins under running water to remove soil, place them on a paper-lined baking sheet, and bake for 40 minutes or until soft. If a wooden toothpick easily goes in, the potatoes are cooked.
• Let cool, remove the skin, cut in pieces, and mash the pulp with a potato masher, collecting it in a large bowl.
• Add sugar and cocoa, mix first using a metal spoon, and then your hands.
• Line a glass bowl with foil with a diameter suitable for your chosen serving dish. Arrange the potato (solid) cream in the bowl, pressing lightly to compact them (you can also prepare them the day before by storing the covered container in the refrigerator).
• Place the metal part of the electric whips and the heavy cream in the freezer for 5 minutes.
• Meanwhile, arrange the serving dish over the bowl, then turn upside down. Lift the bowl, remove the foil, and keep aside.
• Place 45 g sugar, mascarpone, and 1/2 teaspoon natural vanilla essence in a bowl. Cover with cold, heavy cream. Beat with electric whips until well whipped and firm, then transfer to a disposable pastry bag.
• Cover the cake with clumps of whipped cream and decorate with marron glacés crumbs or chocolate chips.
• Look at the cake, thinking you made it, be happy, and serve it. You can also store it in the refrigerator until ready to serve.

Tips

• You can make potato cream a day or two in advance.
• American sweet potatoes (white) are usually available until November. To make this dessert for the holidays or a special event, bake the potatoes and freeze the cooked flesh.