Preheat oven to 350.
Line the bottom of an 8" springform pan with parchment paper and grease the sides.
Add 1/2 cup butter to a small pot or pan over medium heat. Once butter melts, swirl the pan often for a few minutes until it starts to smell nutty. Reduce heat slightly. It will start to foam and brown under the foam. Remove from the heat and let it cool slightly before using in the recipe.
Add bananas to a large bowl and mash well. Add 1/4 cup brown butter, olive oil and sugar and whisk to combine.
Add eggs and vanilla and whisk again.
Add baking powder, cinnamon, salt and flour. Use a spatula to combine.
Add in 1/2 cup pecans and combine well.
Pour batter into the prepared springform and smooth out the top.
Add streusel ingredients to a medium bowl and stir to combine.
Scatter topping onto the cake.
Bake for 40 minutes or until the centre of the cake is fully cooked.
Let the cake cool for 10 minutes before removing the springform. Allow the cake to cool 30 minutes more before slicing.