Preheat oven to 180°C and place muffin cases in a12-hole muffin tin.
Combine dry ingredients in a large bowl.
In a separate bowl, whisk together remaining ingredients, then pour into dry ingredients. Stir with a wooden spoon until just combined.
Spoon batter into liners and bake for 20 minutes or until top springs back when gently tapped. Cool muffins in tin for 5 minutes before transferring to a wire rack to cool completely.
When muffins are completely cool prepare the frosting. Combine cream cheese and icing sugar in a bowl. Beat until smooth and creamy. Place a generous dollop of frosting on top of each muffin. Top with a walnut, if using. Allow frosting to set before serving.