0 Prep the oven and pans: Adjust the oven rack to the middle position and heat the oven to 350°F. Spray two standard loaf pans with nonstick baking spray and line each with a parchment paper sling that covers the bottom and long sides, leaving the short ends uncovered. This makes the loaves easy to lift out after baking.
1 Brown the butter: Melt the butter for the bread in a small saucepan over medium heat, stirring occasionally. When it bubbles steadily and turns golden, don't walk away. Keep stirring until it's a medium-deep golden brown, then immediately take it off the heat. Transfer it to a small bowl and set aside to cool. (I often pop it in the fridge to speed this up.)
2 Mix the crumble: In a small bowl, whisk the flour, brown sugar, cinnamon, and salt for the crumble with a fork. Add the walnuts and whisk once more for a few seconds to coat the nuts, which keeps them from sinking into the loaves.
3 Cut in the butter: Using clean hands, massage the softened butter into the crumble mixture, pressing and smashing the butter with your fingertips as you would for pie dough. When you have coarse crumbs (it won't take long), cover the bowl and refrigerate until the loaves are ready to go in the oven.
4 Mash the bananas: Peel the bananas and add them to a large, shallow bowl or plate. Mash with the back of a fork until you have a thick banana puree.
5 Whisk in the wet ingredients: Whisk the sugars into the mashed bananas, then the vanilla, then the eggs, then the cooled brown butter, then the yogurt, in that order.
6 Mix the dry ingredients: In a separate large mixing bowl, whisk together the gluten-free flour, baking soda, cinnamon, and salt.
7 Combine: Pour the banana mixture into the flour mixture and stir until just combined.
8 Fill the pans: Divide the batter evenly between the two prepared loaf pans. I use a kitchen scale to make sure they're equal.
9 Add the crumble: Top each loaf with the chilled cinnamon crumble, covering the tops as evenly as you can.
10 Bake: Bake for 50 to 70 minutes, until a tester inserted into the center comes out almost clean. It shouldn't be totally clean (a few moist crumbs are fine), and the top should visibly bounce back when you press on it.
11 Cool the loaves in the pans on a wire rack for 10-15 minutes, then lift them out and continue cooling on the rack.
12 Serve warm or at room temperature, preferably with a very cold glass of milk. Enjoy!
