Walnut Cake with American Frosting

1. Melted butter, for brushing pans 
2. 7 ounces all-purpose flour
3. Pinch salt
4. 1 generous teaspoon baking powder
5. 7 tablespoons butter, softened
6. 8 ounces caster sugar (superfine)
7. 1 teaspoon vanilla extract
8. 2 eggs, separated
9. 3 1/2 ounces shelled walnuts, roughly chopped
10. 3 1/2 ounces milk

For the vanilla buttercream filling

1. Preheat the oven to 350 degrees F. Brush 2 (8-inch) or 3 (7-inch) cake pans or sandwich pans with melted butter, and dust with flour. Line the base of each pan with parchment or greaseproof paper. 
2. In a bowl, sift the flour with the salt, and baking powder. 
3. In a large bowl, or in an electric food mixer, cream the butter until soft. Add the sugar, and vanilla extract, and beat until the mixture is light and fluffy. Beat in the egg yolks, and then stir in the chopped walnuts. Fold in a quarter of the flour and milk into the mixture, alternating each, until they are incorporated. 
4. In a separate bowl, whisk the egg whites until they are stiff. 
5. Stir a quarter of the egg whites into the cake mixture, and then gently fold in the rest. Divide the batter between the tins, making a slight hollow in the center of each so that the cake rises evenly rather than forming a peak. 
6. Bake in the oven until firm to the touch or until a skewer inserted into the center of each cake comes out clean, about 16 to 20 minutes. Remove the cakes from the oven and allow to sit for 5 minutes, before turning out onto a wire rack to cool.
7. To make the buttercream filling: Cream the butter until very soft, and then beat in the confectioners' sugar, and the vanilla extract. When the cakes are cool, spread the buttercream filling on one layer, and top with the second layer, sandwiching the layers together. 
8. Make the American frosting as described below. Spread the American frosting quickly over the cake with a palette knife, regularly dipping the knife into a jug of boiling water to help spread the icing. It sets very quickly at this stage, so speed is essential. Scatter with rose petals, if using, or arrange the walnut halves around the top of the cake, and allow the icing to set until it feels dry on the surface.

American Frosting

1. To make the American Frosting: Ensure your cake is ready before you start, as this icing begins to set very quickly. Bring to the boil a saucepan of water large enough to hold a heatproof bowl. Place the egg whites in the bowl and whisk with a hand-held electric beater until very stiff. 
2. In a separate saucepan over a medium-high heat, dissolve the sugar in the water and boil for 5 to 10 minutes until the liquid is thick and syrupy and has reached the 'thread' stage - when the last few drops that fall from a metal spoon dipped into the syrup come off in one long, quite thick and syrupy thread. 
3. Pour the boiling syrup over the stiffly beaten egg whites, whisking all the time with the hand-held beater. Place the bowl in the saucepan of simmering water. Continue to whisk over the water for 10 to 15 minutes until the icing is snow white, very thick and meringue-like. 
4. Spread quickly over the cake with a palette knife, regularly dipping the knife into a jug of boiling water. The icing sets very quickly at this stage, so speed is essential.