Preheat oven to 170 degrees Celsius/360 degrees Fahrenheit
Combine the dry ingredients. Mix the flour with the salt and baking powder in a mixing bowl.
Using a food processor or electric hand whisk, whip the margarine, vanilla essence, and granulated sugar together until smooth and creamy.
Slowly add the eggs until they are fully incorporated but do not overmix.
Lightly fold in the flour mixture, and transfer into 3 lightly greased baking tins.
Bake the sponge for 20 minutes, then remove the sponge to wire racks to cool completely.
Cake Filling
Use the fine grating side of a box grater to grate the white chocolate and roughly chop all of the pistachio nuts.
Whip heavy cream until it thickens, and then fold in roughly a third of the nuts and half of thegrated white chocolate.
Buttercream frosting
Whip the softened butter with sifted powdered sugar until smooth and creamy. Fold in the remaining grated white chocolate.
Assembly
Place one layer of sponge on a cake platter and spread half of the white chocolate and pistachio cream filling over the top. Spread filling right to the edge. Repeat with the second layer of cake, then place the third layer on top.
Use a soft rubber spatula or frosting knife to spread the white chocolate buttercream all over the top and sides of the Italian pistachio cake.
Place the white chocolate buttons on the sides of the cake and sprinkle chopped pistachio nuts across the top.
Using your hands, press more pistachio nuts into the side of the cake to stick to the buttercream surrounding the buttons.
Notes
I use margarine or spreadable butter in this recipe for ease. Otherwise, you can use blocks of butter but take it out of the fridge to soften for an hour before using. To check when the cake is baked, press your finger into the center of the sponge and it should spring back. If it stays indented, bake for another 5 minutes. The edges of the cake should also shrink in from the sides of the tin slightly when done.