Butter a bread pan or a 9-inch square pan. Preheat the oven to 350°F (180°C).
Sift together in a bowl the flour, baking powder, baking soda, salt, and allspice (or cinnamon). Mix in the sugar.
In a large bowl, mix together the butter, egg white, egg, banana puree, buttermilk or sour cream, vanilla and espresso.
Make a well in the center of the flour mixture and stir in the wet ingredients with a spatula until almost mixed. Add in your choice of additions if desired and stir until just combined, but don’t overstir: stop when any traces of flour disappear.
Scrape the mixture into the prepared pan. Bake the bread for 60 minutes or the square cake for 40 minutes. The center must feel lightly-springy and just done. Cool on a baking rack and enjoy with a good cup of coffee.