Heat the oven to 350°F (175°C). Grease a 9×5-inch loaf pan with 1 tbsp softened butter and line it with a parchment sling.
In a large bowl, cream 0.5 cup softened unsalted butter and 1 cup granulated sugar with a hand mixer on medium speed until pale and fluffy, 2–3 minutes.
Beat in 2 large eggs one at a time, mixing well and scraping the bowl after each.
Stir in 1.5 cups mashed very ripe bananas and 1.5 tsp vanilla extract until evenly combined.
In a separate bowl, whisk together 2 cups all-purpose flour, 1 tsp baking soda, and 0.5 tsp fine salt.
Add the dry ingredients to the wet in two additions, folding gently with a spatula just until no dry flour remains; do not overmix.
Scrape the batter into the prepared pan and smooth the top.
Bake 55–65 minutes, until a skewer inserted in the center comes out with a few moist crumbs and the top springs back; tent loosely with foil if the loaf is browning too quickly after 45 minutes.
Cool in the pan for 10 minutes, then lift out and cool on a rack for at least 45 minutes before slicing.