Preheat the oven to 350ºF (180ºC). Grease a standard loaf tin with two tablespoons of butter and set aside.
Add the buckwheat flour, baking powder, baking soda, and salt to a mixing bowl. Whisk to combine and set aside.
Place the butter in a large microwave-safe mixing bowl or a small pot on the stovetop. Melt the butter and cool for 5 minutes.
Whisk the granulated and brown sugars and eggs into the melted butter until smooth.
Add the overripe bananas to the bowl. Use a fork to mash the bananas as much as possible. Stir to combine.
Mix the sour cream and vanilla into the banana mixture. Stir until smooth.
Stir the buckwheat flour mixture into the banana mixture. Pour the batter into the prepared loaf tin. Smooth the top, then sprinkle generously with coarse sugar if using. Bake the loaf for 50 to 55 minutes then transfer the tin to a wire rack for 10 minutes. Turn the loaf out of the tin directly onto the rack and cool completely before slicing and serving.
Notes
Buckwheat banana bread is best served fresh.
Be sure to mash and squish the bananas thoroughly to prevent large mushy pockets in the loaf.
Storage
Room Temperature: Wrap the cooled loaf tightly in plastic wrap and store it at room temperature for up to 3 days.
Freezing: Slice the cooled banana bread and store the slices in an airtight container or freezer bag for up to 2 months. Thaw individual slices at room temperature when ready to enjoy.