Step 1: Preheat oven to 175°C (350°F). Grease a standard 9x5-inch (23x13 cm) loaf pan and line it with parchment paper, leaving overhang.
Step 2: In a large mixing bowl, mash the ripe bananas. Stir in the melted butter, 150g (¾ cup) granulated sugar, and brown sugar until smooth. Mix in the eggs one at a time, then stir in the vanilla extract.
Step 3: In a separate medium bowl, whisk together the all-purpose flour, cream of tartar, baking soda, and salt. Add this dry mix to the wet ingredients and mix just until combined, without overmixing.
Step 4: In a small bowl, combine the 60g (¼ cup) granulated sugar and ground cinnamon for the coating.
Step 5: Spoon about half of the banana bread batter into the prepared loaf pan. Sprinkle about 1 tablespoon of the cinnamon-sugar mixture evenly over the batter. Dollop the remaining batter on top and gently spread it out. Sprinkle the remaining cinnamon-sugar mixture generously over the top surface.
Step 6: Bake for 55–65 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean. If the top browns too quickly, loosely tent with aluminum foil.
Step 7: Let the banana bread cool in the pan for about 10 minutes. Use the parchment paper overhang to carefully lift the loaf out and transfer it to a wire rack to cool completely before slicing.