Preheat oven to 350 degrees. Grease a 9 x 5 pan with nonstick cooking spray or butter.
In a stand mixer with the paddle attachment cream butter and brown sugar until fluffy, 2-3 minutes.
Add in eggs one at a time and mix until combined. Add in vanilla and mashed bananas.
In a medium bowl, whisk together the flour, baking soda, and salt.
With the mixer on low add half of the dry ingredients followed by all of the milk. Mix until just combined then stir in the rest of the dry ingredients and the walnuts. Do not overmix.
Add the batter to the prepared pan and spoon the dulce de leche sauce on top. Swirl the dulce de leche into the bread. (I use slightly warmed dulce de leche to make it easier to spread)
Bake for 1 hour or until a toothpick comes out clean. (If the bread starts to get too brown cover with aluminum foil)
While the bread is still in the pan and slightly warm poke holes in the top of the bread with a toothpick. Drizzle more slightly warmed dulce de leche on top. (I did about 1/4 cup).
Allow the bread to cool completely before releasing from the pan.
Homemade Dulce De Leche Sauce
Place the can of condensed milk in the slow cooker on it's side. You do not need to peel off the label.
Add enough water to completely cover the can.
Cook on low for 8-10 hours. (The longer you cook it the darker / golden brown it will become.)
Remove the can from the slow cooker using tongs and place into cold water to cool before opening. I usually wait about 10-15 minutes before opening.
Store the dulce de leche in an airtight container and enjoy for up to 3 weeks!