Austrian coffee cake recipe with coffee buttercream

Ingredients

• 75g walnuts, finely chopped
• 263g self raising flour
• 2 ½ teaspoons baking powder
• 263g unsalted butter, at room temperature
• 263g golden caster sugar
• 5 eggs
• 2 ½ level tablespoons instant espresso coffee powder mixed with 3 tablespoons - boiling water
• 1 ½ level tablespoons
• 75g Demerara sugar
• 83ml boiling water
• 4 egg whites
• 250g caster sugar
• 250g unsalted butter
• 2 level tablespoon instant espresso coffee powder mixed with slightly less than 1 tablespoon - boiling water