• Preheat the oven to 375f and line a muffin tin with paper cases.
• In a stand mixer, cream the butter and sugar with the vanilla extract.
• Add in the eggs one at a time allowing each to be fully incorporated before adding in the next.
  
• In a separate bowl mix together the flour, salt, baking powder and cinnamon.
  
• Slowly add in the flour mixture alternating with the milk and both starting and ending with the flour.
• Beat on low speed until just combined.
  
• Fold in the diced apple and walnuts.
  
• Using a medium cookie scoop, fill the muffin cases until around three quarters full then place into the oven to bake for around 25 minutes until a toothpick inserted in the centre of a muffin comes out clean.
  
• Cool on a wire rack completely.
  

Now to make the frosting:

• Beat the cream cheese and butter until light and fluffy then add in the vanilla extract and mix until combined.
• Slowly add in the powdered sugar and mix until you have a smooth thick consistency.
• Place the frosting into a piping bag with a simple round nozzle attached and pipe swirls on top of the cooled muffins.
• Sprinkle with extra chopped walnuts if you want an added nutty crunch.