Preheat the oven to 350 F. Butter and lightly flour a 9x4 inch loaf pan, tapping out any excess flour.

In a large bowl, whisk the eggs and packed brown sugar together until pale and fluffy. While whisking, stream in the melted butter or oil, then whisk in cinnamon, nutmeg, ginger, and sea salt.

Mash the bananas with a fork until mostly softened with a few small chunks. Add the mashed bananas and pumpkin puree to the egg mixture and whisk until thoroughly combined.

Sprinkle the flour and baking soda over the batter. Whisk until almost incorporated, then pour in the buttermilk and continue whisking just until a smooth, uniform batter forms. A few banana lumps are fine.

If you like, press one thin, long banana slice gently onto the top of the batter.

Bake in the preheated oven until the top springs back when lightly poked and a tester inserted in the center comes out clean, 50 to 60 minutes.

Cool the loaf in the pan for 5 minutes, then turn it out onto a wire rack and let it cool completely. For the most developed flavor, wrap it tightly and serve the next day.