Walnut cake with coffee frosting

Walnut cake

• 180 g unsalted butter, softened and cut into pieces, plus extra to grease
• 150 g plain flour, plus extra to dust
• 5 eggs, separated
• 1 pinch sea salt
• 100 g walnuts
• 250 g white sugar
• 1 tsp instant granulated coffee
• 2 tsp cocoa powder
• 1 tsp baking powder
• 700 g water

Coffee frosting

• 1 ½ tbsp instant granulated coffee
• 1 tbsp boiling water
• 100 g unsalted butter, softened and cut into pieces
• 200 g icing sugar
• walnuts, chopped, to decorate