For the banana bread: Preheat the oven to 350 degrees F. Line a 12-count muffin tin with cupcake liners.
Sift the flour, baking soda and salt into a medium bowl. Whisk the vanilla and eggs together in a liquid measuring cup with a spout. Set aside.
In a stand mixer fitted with the paddle attachment or with an electric hand-held mixer, cream the sugar and butter until light and fluffy. Gradually pour in the egg mixture while beating; beat until incorporated. Mix in the bananas (the batter will appear curdled, but don't worry), then remove the bowl from the mixer. With a rubber spatula, fold in the flour mixture until just incorporated. Fold in the walnuts, then the Mexican chocolate.
For the cinnamon crumble topping: In a small bowl, mix together the flour, brown sugar and cinnamon. Cut in the butter with 2 knives until uniform.
Fill the cups of the muffin tin about halfway with the batter. Spoon the cinnamon crumble topping over the muffins, pressing it lightly into the batter. Bake until the topping is golden and a toothpick inserted in the center of the muffins comes out clean, 20 to 25 minutes. Let cool before serving. These muffins are great with a smear of butter.
To bake as a loaf without the crumble topping, pour the banana bread batter into a buttered 9-by-5-by-3-inch loaf pan. Bake at 350 degrees F until a toothpick inserted in the center of the bread comes out clean, about 55 minutes. Cool the bread in the pan on a wire rack for 5 minutes, then turn the bread out of the pan onto the rack to cool completely.