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.
In a bowl, sift
the flour with the salt, and baking powder.
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.
In a separate bowl, whisk the egg whites until they are stiff.
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.
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.
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.
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.