Walnut and coffee cream muffin

• 75 g walnuts, roughly chopped
• 125 g unsalted butter, room temperature, cut into pieces (2 cm)
• 125 g milk
• 180 g natural yoghurt
• 1 egg
• 1 tsp vanilla bean extract
• 1 tsp instant granulated coffee
• 300 g self-raising flour
• 125 g caster sugar

Buttercream icing

• 1 tbsp boiling water
• 2 tbsp instant granulated coffee
• 175 g butter, room temperature, cut into pieces (2 cm)
• 190 g icing sugar
• 2 tbsp chopped walnuts, to decorate
• 1 tbsp milk chocolate chips, to decorate