Whisk together the egg yolks and half the sugar. When the eggs are pale in colour, add your spices and vanilla. Whisk again until fully incorporated.
Sift the flour over the egg yolk mixture, set aside.
In a separate bowl, whisk the egg whites and cream of tartar until it has tripled in volume, then add the remaining sugar. Carry on whisking until the mixture forms soft peaks.
Fold in the yolk mixture a spoon at a time, being careful not to lose any air. Gently scrape the mixture into the tin, levelling the top with a spatula.
Bake for 35 minutes or until a skewer comes out clean and the cake feels springy.
Remove from the oven, turn upside down and balance on the neck of a bottle for about one and a half hours, a long-necked wine bottle is perfect for this. To remove the cake from the tin slide a thin knife around the sides and gently ease the cake out from the tin.
To make the butterscotch icing: half fill a saucepan with water and bring it to a simmer, place a glass bowl over the water and add the butter, sugar and cream, whisk until melted and smooth. Take it off the heat and let it cool. Add the vanilla and sift in the icing sugar. Whisk until smooth.
To make the caramel shards: firstly you need a heavy bottomed pan and a thermometer. Place the sugar into the pan over a high heat and bring to the boil, add the thermometer, do not stir when boiling as that can cause crystals to form. Once the thermometer reaches 190˚C or 374˚F take it off the heat. Spread the caramel on to a sheet of parchment so it becomes a thin layer. Leave to set.
Take your cake, using a pallet knife spread the butterscotch icing all over the cake ensuring you coat the middle hole too as the cake can dry out.
Lift the caramel from the parchment and break into shards and decorate as you choose!