Generously grease a 12-cup bundt pan. Preheat oven to 350°F.
In a medium bowl, stir together ¼ cup of sugar, the pecans, and the cinnamon. Sprinkle half of the mixture into the bottom of the bundt pan.
Place the butter and remaining 1 cup of sugar in the bowl of a stand mixer. Mix on medium until light and fluffy. Add the eggs and mix until combined. Add the mashed banana, sour cream, and vanilla, and mix just until combined.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Fold the flour mixture into the banana mixture—just until combined.
Pour half of the batter into the bundt pan, then sprinkle the remaining pecan mixture on top. Top with the remaining batter.
Bake for 40–45 minutes, or until a toothpick inserted into the center comes out clean.
Let cool for 10 minutes, then gently run a butter knife around the edges (as best you can with the fluting). Promptly invert the cake onto a wire rack to finish cooling.
Notes
Be sure to use overripe bananas (with at least some brown spots) for the best banana flavor. Remove the cake from the pan 10 minutes after removing it from the oven. This ensures that it'll remove more cleanly since the sugar hasn't hardened on the bottom. Store the cake in an airtight container for up to 5 days. You can also freeze the cake for up to 3 months, wrapped in foil and then in an airtight container or resealable freezer bag.