Microwave 4 of the bananas in a microwave safe bowl for 4 minutes.
Drain the bananas. You should get at least 1/2 cup of liquid. A little more or less is fine. You need very ripe bananas for this to work well.
Put the banana liquid and bourbon in a small saucepan and simmer until reduced. This can take up to 5 minutes, you are looking for it to thicken slightly and go down (reduce) by about half. The alcohol will cook off quickly, so don't worry about getting this perfect.
Mix wet ingredients; egg, bananas, bourbon/banana liquid, butter, vanilla, and brown sugar.
In a separate bowl, mix dry ingredients. Flour, baking soda, baking powder, salt. It is important to get these mixed well to avoid a mouthful of baking powder or soda.
Mix the wet and dry ingredients until just combined.
Pour batter in a well greased or parchment-lined loaf pan.
Take your 5th banana and cut it in half. Cut each half lengthwise to create 4 longer pieces of banana.
Lay the 4 pieces of banana across the top of your loaf, inside facing up.
Sprinkle the granulated sugar over the cut bananas so they will caramelize while baking.
Bake at 350℉ for about 50 minutes. Cooking time will vary based on oven temperature variation. It is ready when the top is golden, no batter is visible on top, and a skewer comes out clean.
Let cool one minute, remove from pan and cool on a cooling rack for about 30 minutes to an hour.