Center a rack in the oven and preheat it to 350°F (176°C). Generously butter a 9- to 10-inch (23 cm or 25 cm) Bundt pan (12-cup capacity). If you have a silicone Bundt pan, there’s no need to butter it. Do not place the pan on a baking sheet, as you want the oven’s heat to circulate through the Bundt's inner tube.
Whisk together the flour, baking soda, and salt.
Using a stand mixer fitted with a paddle attachment or a hand mixer in a large bowl, beat the butter until creamy. Add the sugar and beat at medium speed until pale and fluffy. Mix in the vanilla, then add the eggs one at a time, beating for about 1 minute after each addition.
Reduce the mixer speed to low and mix in the bananas. Next, add half of the dry ingredients (don’t be alarmed if the batter curdles), followed by all the sour cream, and then the remaining flour mixture. Scrape the batter into the pan, rap the pan on the counter to release any air bubbles, and smooth the top.
Bake for 65 to 75 minutes, or until a thin knife inserted deep into the center of the cake comes out clean. Check the cake after about 30 minutes; if it is browning too quickly, cover it loosely with a foil tent. Transfer the cake to a rack and cool for 10 minutes before unmolding it onto the rack to cool completely to room temperature.
If you have the time, wrap the cooled cake in plastic wrap and let it sit on the counter overnight before serving—it’s even better the next day.