Preheat the oven to 180°C (350°F). Line a 12-cup muffin pan with liners.
Add all the dry ingredients to a large bowl and mix until well combined and evenly distributed.
Add all the wet ingredients (including the optional walnuts) and mix until just combined.
Use an ice cream scoop or a spoon to divide the batter into your muffin pan. The muffin batter should be close to the top of each cavity.
Add all ingredients for your crumb topping to a medium bowl. Mix with a spoon or your fingertips until the mixture is combined, flaky and crumbly.
Generously sprinkle the crumb topping onto your muffins.
Bake the muffins for 20-25 minutes or until you can insert a toothpick into one and it comes out clean.
Allow the muffins to cool in the muffin pan for 15 minutes then place them on a cooling rack.
Enjoy the muffins warm or at room temperature. Store any leftovers in an airtight container.
