Preheat oven to 325°F. Line a standard muffin tin with paper liners.: When the oven warms to 325°F , you should feel a gentle heat if you open the door briefly, and the air will smell faintly warm and dry. This moderate temperature encourages an even rise and a golden top without overly fast browning. Preheating is important because putting batter into a cold oven can yield flat, dense muffins. A common error is neglecting liners or not preparing the pan, which can make removal sticky and messy.
In a medium bowl, whisk together the flour, baking soda, cinnamon and salt.: As you whisk, you will notice a floury aroma with warm spice notes from the cinnamon , and the mixture should be evenly colored with no visible clumps. Properly combining these dry components ensures even distribution of leavening so some muffins do not rise more than others. Avoid compressing the mixture in the bowl which can lead to uneven measuring later.
In the bowl of a stand mixer, or in a large bowl with an electric mixer, combine olive oil and sugars until thoroughly combined.: The sugars should dissolve slightly into the olive oil , creating a glossy, grainy mixture that smells sugary and faintly fruity. This step helps dissolve the sugar crystals, contributing to a tender texture and smooth crumb. If the sugar does not incorporate well, your muffins may have gritty spots, so mix until the grains are mostly blended.
Beat in eggs until incorporated and stir in bananas, vanilla and water.: Once the eggs are worked in, the mixture will look creamier and slightly puffier, and adding mashed bananas will introduce a rich, fruity aroma. The vanilla will deepen the scent profile, and the hot water will relax the batter for a smooth consistency. Avoid overbeating after the eggs are added, because too much air can make the muffins collapse as they bake.
Add dry ingredients to the wet ingredients and mix until just combined and gently stir in chopped nuts.: You should see streaks blending away, with the batter becoming uniform but still thick and slightly lumpy from the bananas . Stirring walnuts in at the end preserves their crunch and prevents them from sinking. A common mistake is overmixing, which develops gluten and results in tough muffins, so stop when the dry patches are gone.
Use a large cookie scoop to evenly distribute the batter between the prepared muffin wells.: Scooping gives uniform size and consistent baking, and you will notice the muffin wells filling to a pleasing dome shape when properly portioned. A level scoop ensures the tops bake evenly so some muffins do not finish before others. If you eyeball portions, you risk uneven bake times and inconsistent texture.
Stir all ingredients together in a small bowl. Make sure the olive oil is thoroughly incorporated.: For the crumble, mixing will yield a slightly clumpy, sandy texture with visible oats and chopped walnuts . The olive oil should bind the sugars and flour into a cohesive topping that will crisp in the oven. If the topping seems too dry it will not adhere, and if too wet it will not crisp properly, so adjust by ensuring thorough incorporation rather than adding extra liquid.
Sprinkle each muffin with the crumble topping, pressing it in gently.: Pressing the topping ensures it sticks and bakes into a compact, crackly layer that contrasts with the soft interior. You should see a textured surface that will brown and smell nutty as it bakes. Resist the urge to overload each muffin with crumble, because an excessive topping can slide off or burn.
Bake in preheated oven for 20-23 minutes, or until the muffins are golden brown and slightly firm to the touch.: As they bake, the kitchen will fill with a warm, sweet aroma and the tops will transition to a golden brown while the centers set. A light press on the top should spring back gently, indicating doneness. A common pitfall is removing them too early which results in a gummy center, so rely on the color and spring test rather than time alone.
Allow to cool for 5 minutes in the pan, then remove to a cooling rack and allow to cool completely.: Cooling briefly in the pan helps the muffins settle and makes removal easier, after which transfer to a rack for airflow to prevent sogginess. You will notice steam stop rising from the muffins as they cool, and the texture will firm slightly. Cutting or storing them too hot traps steam and can make them dense, so be patient for best texture.
