To make the walnut streusel, add the brown sugar, flour, cinnamon (if using) and cold butter to a small mixing bowl, and using your fingers, cut the butter into the flour mixture until it resembles coarse crumbs. Stir through the walnuts and set aside in the fridge until ready to bake the muffins.

Prepare the feijoa by scooping out all the feijoa flesh and discarding the skins. Lightly mash the pulp and set aside.

Preheat the oven to 180C and line two, 6-cup muffin pans or one, 12-cup muffin pan with muffin liners. Set aside.

To make the muffin batter, whisk together the eggs, milk, vanilla and cooled, melted butter into a small bowl. Add the feijoa pulp and mix gently. Set aside.

In a larger bowl, mix together the flour, brown sugar, baking powder, baking soda and cinnamon. Add the wet ingredients and fold gently until just combined and there are no streaks of flour. Expect a thick batter!

Spoon the batter into the prepared muffin pans using an ice cream scoop, ¼ cup or a spoon. Fill the muffin liners to just over ¾ full then top each one with a tablespoon of the walnut streusel mixture.

Bake in the preheated oven for 20-25 minutes, or until a cake skewer or toothpick inserted in the centre comes out clean (making sure to not pierce a piece of feijoa). Allow the muffins to cool in the pans for five minutes before transferring to a wire rack and allowing to cool completely.

These Feijoa and Walnut Streusel Muffins keep covered at room temperature for 2 days, or covered and in the fridge for 4 days.