Preheat the oven to 350° Fahrenheit and grease a bread pan with butter or cooking spray.
Make the brigadeiro! To do this, melt the butter in a medium saucepan over low heat. Then add the condensed milk and cocoa powder and stir to combine. Continue stirring regularly as the mixture begins to thicken. Keep stirring/cooking until it's fairly thick and when you draga spatula through it you can see the bottom of the pot for a couple seconds before it comes together again. Once you've reached this point, remove from heat and set to the side.
Now make the banana bread batter. Cream together the butter and sugar. I like to use a stand mixer for this but you can also do it by hand or with a hand mixer.
Add the eggs, mashed banana and vanilla extract to the butter and sugar and mix to combine.
In a medium sized bowl, sift together the flour, baking soda, and salt. Then add this dry mixture to the wet one and stir until just combined, being careful not to overmix.
Pour half the batter into the greased pan. Then add about 4 tbsp brigadeiro and swirl with a toothpick or a knife. Next, add the rest of the batter, followed by another 4 tbsp brigadeiro and another swirling. (Note: you can use more or less brigadeiro than this calls for based on preference. If you follow this exactly, there will be a bit extra so you've got wiggle room.)
Bake for about 1 hour and 15 minutes, or until the top is brown and shiny and when you place a toothpick through the middle it emerges clean. Then remove from the oven and move to a cooling rack for a bit before slicing!