Grease and flour a baking pan.

Add the melted butter, stirring in a circular motion to combine. Then add the eggs, sugar, walnuts, and vanilla extract.

Preheat the oven. In a larger bowl, sift the flour and baking powder together. Add the banana mixture and mix until well combined. Pour into the prepared pan and bake at 350°F (180°C) for 45-50 minutes. After this time, insert a knife to check if it's done; if it comes out clean, it's ready.