Preheat your oven to 350°F (175°C).
Grease a 9x5-inch loaf pan or line it with parchment paper.
In a separate bowl, whisk together the flour, baking soda, and salt.
In another large bowl, mix the melted butter, brown sugar, and egg until smooth.
Add the mashed frozen bananas and vanilla extract to the wet mixture, stirring well.
Gradually add the dry ingredients into the wet ingredients, stirring until just combined.
Pour the batter into the prepared loaf pan and spread it evenly.
Bake for 60-70 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven and let it cool in the pan for 10 minutes.
Transfer to a wire rack to cool completely before slicing.