Preheat your oven to 350°F (175°C). Grease a 9x5-inch light metal loaf pan and line it with parchment paper, leaving a 2-inch overhang on the long sides.
In the bowl of a stand mixer fitted with the paddle attachment (or using a hand mixer), beat the cold butter and granulated sugar on medium-high for 5 minutes, scraping down the sides once. The mixture should look pale, fluffy, and slightly granular — like damp sand clumping together.
Add the eggs one at a time, beating on medium for 30 seconds after each. Scrape down the bowl. The mixture may look slightly curdled — that's fine.
Beat in the yogurt and vanilla on low until just combined. The mixture will smooth out.
In a separate bowl, whisk together the flour, baking soda, baking powder, and salt.
Add the dry ingredients to the wet in three additions, folding gently with a rubber spatula. Stop as soon as no flour streaks remain. Overmixing ruins the velvet crumb.
Fold in the mashed bananas in two additions, just until evenly distributed. A few small streaks of banana are fine — they'll create concentrated flavor pockets. If using add-ins (walnuts or chocolate chips), fold them in now. The batter should be thick and scoopable, not runny.
Pour the batter into the prepared pan and smooth the top. Bake for 55–65 minutes, rotating the pan halfway through. At the 45-minute mark, tent the loaf with foil if the top is browning too quickly. A wooden skewer inserted into the center should come out with a few moist crumbs — not clean, not wet.
Let the loaf cool in the pan on a wire rack for 20 minutes. Then use the parchment sling to lift it out and cool completely on the rack before slicing. Slicing warm results in a crumbly, gummy mess — resist the urge.