Step 1

Preheat the oven to 350°F (177°C) and line a 9x5-inch (23x13cm) loaf pan with parchment paper. Set aside.

Step 2

In a large mixing bowl with an electric mixer with a whisk or paddle attachment, beat the butter, oil, and brown sugar on medium-high speed for about 2-3 minutes until light, creamy, and pale.

Step 3

Add the bananas and mix for about 1 minute. Stir in eggs and vanilla until everything is well mixed, about 1-2 minutes. Add the sourdough discard and mix to combine. The mixture will look curdled, but that's perfectly normal.

Step 4

Then add the flour, baking soda, and salt and mix on low speed just to incorporate about 1 minute. If used, fold in the chocolate and chopped banana.

Step 5

Pour the batter into the prepared baking pan, spread evenly, top with halved banana slices and brown sugar if desired, and bake for 55-65 minutes, or until a toothpick comes out clean in the middle.

Step 6

Remove from the oven and let cool for 15 minutes. Then remove from the pan and transfer to a cooling rack and let cool completely. Store in an airtight container at room temperature for up to 4 days or freeze for up to 3 months.