Preheat the oven to 175°C (350°F) and lightly grease or line a 9x5-inch loaf pan with parchment paper.

In a small bowl, mix 40 g granulated sugar and 5 g ground cinnamon to make the cinnamon sugar topping and set aside.

In a large mixing bowl, whisk together the melted butter, 150 g granulated sugar, and 50 g light brown sugar until combined.

Add the mashed bananas, eggs, and vanilla extract to the sugar mixture and whisk until smooth and well blended.

In a separate bowl, whisk together the flour, baking soda, salt, and 3 g ground cinnamon until evenly combined.

Add the dry ingredients to the wet ingredients and gently stir with a spatula until just combined and no dry streaks remain.

Fold in the sour cream or yogurt until the batter is smooth, being careful not to overmix.

Pour half of the batter into the prepared loaf pan and smooth the top with a spatula.

Sprinkle about half of the cinnamon sugar topping evenly over the first layer of batter.

Pour the remaining batter over the cinnamon sugar layer and gently spread it to the edges.

Sprinkle the remaining cinnamon sugar and optional coarse sugar evenly over the top of the batter.

Drizzle 15 g melted butter over the top to help the cinnamon sugar form a crisp crust.

Bake the loaf for 55–65 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs.

If the top is browning too quickly before the center is done, loosely tent the loaf with foil during the last 15 minutes of baking.

Remove the pan from the oven and let the banana bread cool in the pan on a cooling rack for 15–20 minutes.

Run a butter knife around the edges, carefully lift or invert the loaf from the pan, and let it cool on the rack until just warm before slicing and serving.