Preheat the oven to 356 °F (176°C). Grease and line a 9x5-inch (23 x 16cm) loaf pan.
Melt the butter in a large bowl. Add in the bananas and mash them well. Add the sugar, vanilla, egg, and molasses and whisk until combined.
Whisk together the flour, spices, salt, and baking soda in a separate bowl.
Add the dry ingredients to the banana mixture and whisk together until combined.
Pour over the hot water and stir it carefully until combined into a pourable cake batter. There will be lumps from the mashed banana and that's okay.
Pour the batter into a prepared loaf pan.
Slice the extra banana lengthwise and place the sliced banana on top of the batter. Sprinkle with a little turbinado sugar.
Bake in a preheated oven for approximately 60 minutes until cooked through and a knife inserted in the loaf comes out clean.
Remove the loaf from the oven and allow it to cool in the pan for 5 minutes.
Lift it from the pan and allow it to cool further on a wire rack for at least 20 minutes before slicing.
Notes
*The cup sizes given are for US cups. Note that these are smaller than metric cup sizes. For best results, use grams.The banana gingerbread stays fresh at room temperature for up to 3 days. It can also be wrapped tightly and frozen for up to three months.