Preheat the oven to 350. Grease a 9” round spring-form pan and cut a circle of parchment paper for the bottom of the pan.
Prepare flax egg by whisking flax and water together. Set aside to gel for a minimum 10 minutes.
In a medium large bowl mash bananas until there are no lumps. Add in sweetener, cinnamon, nutmeg, salt, vanilla, milk and olive oil and whisk until everything is well mixed.
Add dry ingredients and mix well. Add in chopped walnuts after batter is well mixed. Scrape batter into prepared pan and tap on the counter a few times to release air bubbles.
For the topping, place buckwheat flour, coconut sugar, salt and cinnamon and stir to combine. Using two knives or a pastry cutter cut cold butter into the flour mixture. Once butter is in smaller pieces use clean hands to combine butter. You are looking for a crumb-like texture. Spread crumb on top of cake and place in a preheated oven. Bake cake for 50-55 minutes until a toothpick comes out clean.
Allow the cake to rest in a round pan for 15 minutes before gently removing spring form. Once the cake has fully cooled you can gently remove the bottom of the pan and slide onto the cake stand of plate to cut. I served mine with strawberries and raspberries. Keeps well at room temperature for a couple of days.
*Macros are provided as an estimate for convenience. Your final numbers will vary depending on ingredient brands, substitutions, and how you portion the recipe.