Preheat your oven to 350°F (175°C). This ensures that your banana bread will bake evenly.
Spray the loaf pan with cooking spray or line it with parchment paper to prevent the bread from sticking.
In a large mixing bowl, beat together the softened butter and sugar until light and fluffy. This should take about 2-3 minutes.
Add the mashed bananas, skim milk, lite sour cream, egg whites, and vanilla extract to the butter-sugar mixture. Mix until well combined.
In a separate bowl, whisk together the unbleached all-purpose flour, whole wheat flour, ground flaxseed, baking soda, and salt.
Gradually add the dry ingredient mixture to the wet ingredients, stirring gently until just combined. Be careful not to overmix; it’s okay if there are a few lumps.
Pour the batter into the prepared loaf pan, smoothing the top with a spatula.
Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean. Keep an eye on it towards the end of baking time to avoid overbaking.
Once baked, remove the bread from the oven and let it cool in the pan for about 10 minutes. Then, transfer it to a cooling rack to cool completely.
In a double boiler or microwave-safe bowl, melt the semisweet chocolate and shortening together until smooth. Stir until fully combined.
Once the banana bread has cooled, drizzle the chocolate glaze over the top. Allow the glaze to set for a few minutes before slicing.