Preheat your oven to 180 degrees celsius.: When the oven reaches temperature you will notice the air feels steadily warm and a faint dry scent of heated air emerges, which ensures the batter begins rising the moment it hits the tin. Preheating is important for proper oven spring and even baking. A common mistake is putting batter into a cold oven, which leads to pale, dense tops and longer bake times, so always wait until the oven indicator is stable.
Scoop out the flesh of the feijoas into a medium sized bowl and mash roughly with a fork.: The mashed feijoa flesh should look pulpy with some small chunks remaining, releasing a heady, floral aroma that signals ripeness. The texture informs moisture content, and leaving small pieces gives bursts of fruity pockets in the crumb. Avoid over pureeing, which can make the batter too liquefied and alter rise and texture.
Add, oil, egg, juice and zest of lemon, and vanilla to the mashed feijoa. Combine well with a whisk or fork.: After combining you will notice the mixture glosses slightly from the olive oil , and the lemon scent will brighten the feijoa fragrance. Whisking creates a cohesive emulsion so the wet ingredients distribute evenly through the dry mix. If you skip whisking thoroughly you may find streaks of oil or clumps of egg white, which leads to uneven baking.
In a second larger bowl, combine the dry ingredients: flour, baking powder, brown sugar and coconut. I do this with a whisk to ensure they are combined and any lumps of brown sugar or coconut are broken up.: Whisking these dry items together removes lumps and aerates the mix, and you should see a uniform pale, sandy texture with small flecks of coconut. Breaking up brown sugar clumps is key so chunks do not create dense pockets. Do not over whisk, as overworking the flour can develop gluten early and toughen the muffins.
Add the wet ingredients to the dry ingredients. Gently combine with a wooden spoon or spatula to make the feijoa muffin batter.: As you fold the wet into the dry the batter will come together into a slightly thick, speckled mixture smelling faintly floral and citrusy, with visible flecks of coconut. Folding rather than vigorous stirring protects the tender crumb by minimizing gluten development. A common error is overmixing until smooth, which tightens the crumb and makes the muffins tough instead of light.
Spoon the feijoa muffin batter into a 12 portion muffin tin. I line mine with a greaseproof bake cup.: The batter should be thick enough to mound slightly on the spoon and hold shape as you drop it into each cavity, producing uniform tops and even bake times. Lining with greaseproof paper or muffin cups helps with release and presentation. Beware of underfilling or overfilling, which leads to uneven rise, so aim for about three quarters full for a nice domed top.
Optional extra: scoop the flesh from a couple of feijoas and slice thinly. Gently push into the top of the muffin and sprinkle with a little brown sugar.: Gently pressing these thin slices into the batter's surface before baking will caramelize the edges and give an attractive, fragrant topping that signals the fruit inside. The thin slices will glisten and take on a slight golden edge as they bake, adding texture and visual appeal. Avoid thick slices which can sink and make the center undercooked.
Bake at 180 degrees for approximately 25 minutes.: During baking you will hear a faint dry settling sound and smell a sweet, toasty aroma as the sugar caramelizes and the coconut toasts lightly. The tops should turn golden and spring back when lightly pressed, with a toothpick inserted into the center coming out clean or with moist crumbs. A common troubleshooting tip is to rotate the tray halfway if your oven has hot spots; underbaking results in gummy centers while overbaking yields dry, crumbly muffins.