Position a rack in the center of the oven and preheat the oven to 400°F. Line a muffin tin with six paper liners.
Line a quarter sheet pan with parchment paper. Spread the walnuts in a single layer across the pan. Bake for 5 to 10 minutes, or until fragrant and toasted brown. Use a heatproof rubber spatula to toss the nuts every 2 to 3 minutes to ensure even toasting. Once toasted, scrape the nuts onto a plate to prevent them from cooking further.
In a medium bowl, use a fork to mash the bananas, adding the sour cream and vanilla and stirring them into the mixture as you do. The mixture will be thick and chunky, but that's totally okay, I promise!
In another medium bowl, whisk together the flour, baking soda, baking powder, and salt.
In the bowl of a stand mixer fitted with the paddle attachment, combine the butter and sugar. Beat on medium until light, fluffy, and doubled in volume, 2 to 3 minutes, using a rubber spatula to scrape down the bottom and sides of the bowl as necessary. Reduce the mixer to low and add the egg, mixing until combined. Scrape down the bottom and sides of the bowl. With the mixer on low, add the dry ingredients in two equal parts, adding the second half only when the first half has been just combined. Add the banana mixture all at once and beat on low until just combined. Add ¾ cup of the walnuts and beat until evenly distributed. Scrape down the bottom and sides of the bowl once more, and beat on low for an additional 30 seconds. Use a 1-tablespoon or 3-tablespoon cookie dough scoop to fill each lined cavity with 6 tablespoons of the batter. Sprinkle the top of each muffin, aiming for the batter and avoiding the pan, with the remaining walnuts. Pour warm water into the empty cavities of the muffin tin, filling them at least ⅔-rds of the way up. Bake for 22 minutes, or until the muffins are domed and golden brown around the edges. A skewer inserted into the center of a muffin should come out with few crumbs attached. Cool in the muffin tin on a wire rack for 10 minutes. then turn the muffins out onto the rack. Serve warm or at room temperature. The muffins can be wrapped individually in plastic wrap and stored at room temperature for up to 2 days.