Popcorn and peanut butter chocolate cake

Ingredients

• 1/2 cup (50g) cocoa powder
• 1/2 cup (125ml) boiling water
• 185 grams butter, softened, chopped
• 1 1/2 cup (330g) caster sugar
• 2 teaspoons vanilla extract
• 3 eggs
• 1 1/2 cup (225g) self-raising flour
• 1/2 cup (75g) plain flour
• 3/4 cup (180ml) buttermilk
• 2 cups (60g) nutty caramel popcorn
• 2 teaspoons icing sugar
• Peanut butter frosting
• 435 grams ready-made vanilla frosting
• 200 grams smooth peanut butter
• 1 tablespoon milk