Savoy cakes are known for being particularly light and this is no exception. Flavoured with chopped roasted pecans and caramel, it’s decorated with chocolate clock faces, and fondant clouds and balloons.
Heat the oven to 180°C/160°C fan/350°F/Gas 4. Prepare the tin. Brush the sides and base of the already greased and lined tin with melted butter. Sift the icing sugar and potato flour into a small bowl, then sift a second time into the tin. Swirl to coat, then tap out the excess.
Make the sponge. Tip the pecans into a baking dish and toast in the oven for about 10 minutes, until lightly coloured. Leave to cool then chop to medium-fine.
Put the egg whites into the bowl of a stand mixer fitted with the whisk. Add the cream of tartar and whisk to soft peaks. Whisk in half the sugar to stiff peaks, then carefully transfer to a clean bowl and set aside.
Put the egg yolks in the mixer bowl. Add the caramel flavouring and the remaining sugar and whisk until the mixture is very thick and the whisk leaves a ribbon-like trail when lifted.
Sift the plain flour and potato flour into another bowl, then sift one third into the egg yolk mixture and gently fold in. Repeat twice more, then fold in the egg whites in 3 batches, adding the chopped pecans with the final batch. Avoid over-mixing.
When just combined, transfer the mixture to the prepared tin. Spread evenly, then dust the top heavily with golden icing sugar.
Bake for about 32–35 minutes, until well risen, golden brown and springy when pressed. Transfer to a wire rack, loosen the sponge, unclip the tin and gently remove the sponge. Leave to cool.
To make the chocolate clock faces, gently melt the white chocolate in a small, heatproof bowl suspended over a pan of steaming hot (but not boiling) water. Alternatively, melt it in the microwave in 10-second bursts.
Draw 4 circles of 6cm diameter on the underside of the baking paper lining the baking sheet and spread the white chocolate just inside the lines to make neat, flat discs about 3mm thick. Leave to set.
Melt the dark chocolate as before and spoon into the small piping bag. Snip the end and pipe a clock face and hands on each disc. Attach the clocks at regular intervals to the sides of the cake with small blobs of chocolate.
To make the fondant decorations, sift the fondant icing sugar into a bowl, then mix with 2 teaspoons of water to a firm fondant paste. Turn out onto a worktop dusted with icing sugar and knead lightly until smooth.
Dust a rolling pin with more icing sugar and roll out the icing to about 4mm thick. Using a sharp knife, cut out balloon and cloud shapes. You’ll need 2 bunches of 5–7 balloons and 2 clouds.
Using the food-colouring paints and the cake-decorating paintbrush, paint the balloons red, green, blue, yellow and violet, and paint blue around the edges of the clouds. Finish with edible glitter (if using).
Attach the balloons to the cake between the clock faces with dabs of melted chocolate, then pipe on balloon strings with the melted dark chocolate. Best eaten the same day.