Preheat your oven to 350°F (175°C). Grease a 9×5-inch loaf pan or line it with parchment paper to prevent sticking.
In a large bowl, combine the mashed bananas and melted butter. Mix well.
Stir in the vanilla extract and the egg until the mixture is smooth.
Add the granulated sugar, brown sugar, baking soda, salt, cream of tartar, and cinnamon. Mix everything until it’s well combined.
Gently fold in the all-purpose flour until just combined. Be careful not to overmix.
In a small bowl, mix the 2 tablespoons of granulated sugar and 1 teaspoon of cinnamon for the topping.
Pour half of the batter into the prepared loaf pan. Sprinkle half of the cinnamon-sugar mixture on top. Then, add the remaining batter and finish with the rest of the topping.
Bake in your preheated oven for 55–65 minutes or until a toothpick inserted into the center comes out clean.
Let the bread cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely before slicing.
Slice and enjoy your Snickerdoodle Banana Bread!