Banana Muffins With Brown Sugar Oat Streusel

Name: Banana Muffins With Brown Sugar Oat Streusel
Ingredients: ½ cup (100 g) dark brown sugar; ¼ cup (31 g) unbleached all-purpose flour; Pinch kosher salt; 2 Tbsp. rolled oats; 2 Tbsp. unsalted butter, at room temperature; Neutral oil (for muffin pan); ½ cup (1 stick) unsalted butter; 2¼ cups (281 g) unbleached all-purpose flour; 2 tsp. baking powder; 1 tsp. baking soda; 1 tsp. ground cinnamon; ½ tsp. kosher salt; ¼ tsp. freshly grated nutmeg; 2 large eggs; 1 cup mashed ripe banana (from about 2 large or 3 small), plus 1 ripe banana, coarsely chopped (optional); ¾ cup plain whole-milk yogurt (not Greek); 1 tsp. vanilla extract; ½ cup (64 g) toasted walnuts, coarsely chopped (optional)