In a large bowl, whisk together the flour, sugar, baking powder, and salt.
In another bowl, mix the mashed bananas, eggs, butter, vanilla extract, and milk until well combined.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined.
Preheat your Ninja Speedi according to the manufacturer’s instructions for baking.
Grease the baking pan or insert a parchment paper liner.
Pour the batter into the prepared pan, spreading it evenly.
Place the pan in the Ninja Speedi and bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the Ninja Speedi and let it cool in the pan for about 10 minutes.
Transfer the cake to a wire rack to cool completely.
Once cooled, slice and enjoy your delicious banana cake!