Ingredients
Method
In a medium bowl, mix 1/2 cup (100 g) dark brown sugar, 1/4 cup (31 g) unbleached all-purpose flour, and a pinch kosher salt to combine. Add 2 Tbsp. rolled oats and 2 Tbsp. unsalted butter, at room temperature and cut them in with a fork or your fingers until the mixture begins to clump together. When pressed, the streusel should clump together in your fist but crumble into nice lumps. Mix a bit longer, if needed. Transfer to a small container and refrigerate until ready to use. Do ahead: Streusel can be made up to 1 month ahead and frozen in an airtight container, or up to 2 weeks ahead and refrigerated in an airtight container.
Preheat oven to 350º. Line a standard 12-cup muffin pan with paper liners. Lightly coat flat top of muffin pan with neutral oil to prevent sticking.
Heat ½ cup (1 stick) unsalted butter in a medium saucepan over medium heat, stirring often, until it foams, then browns, 5–8 minutes. Scrape into a large bowl and immediately whisk in ½ cup (100 g) dark brown sugar to combine. Cool to room temperature.
Whisk together 2¼ cups (281 g) unbleached all-purpose flour, 2 tsp. baking powder, 1 tsp. baking soda, 1 tsp. ground cinnamon, ½ tsp. kosher salt, and ¼ tsp. freshly grated nutmeg in a medium bowl.
Add 2 large eggs to the brown butter mixture and whisk until smooth. Whisk in 1 cup mashed ripe banana (from about 2 large or 3 small), ¾ cup plain whole-milk yogurt (not Greek), and 1 tsp. vanilla extract.
Fold dry ingredients into wet ingredients until halfway combined. Add 1 ripe banana, coarsely chopped and ½ cup (64 g) toasted walnuts, coarsely chopped, if using, and mix until just combined and no flour streaks remain.
Divide batter evenly among muffin cups, filling them to the brim. Top each with 1 tablespoon (heaping) streusel and lightly press with your hands to adhere. (You may have excess streusel.)
Bake muffins, rotating pan after 15 minutes, until the top springs back to the touch or a tester inserted into the center comes out clean, 25–30 minutes. Let sit in pan until cool enough to handle, about 3 minutes, then transfer to a wire rack to cool completely. Muffins are best the day they are made and even better if they are still warm.