• Preheat oven to 350°F. Spray the wells of a standard size muffin pan with nonstick spray, or line with muffin liners.
• Chop 1 cup of walnuts. Place ¾ cup of them on a small sheet pan and toast in a 350°F oven for 5 minutes. Increase oven heat to 425°F.

Make Streusel

• Further chop the remaining ¼ cup of walnuts until very fine, almost flour-like. Add to a small bowl with flour, brown sugar, ½ teaspoon of cinnamon, and a pinch of salt. Add 4 tablespoons of cold, cubed butter. Work together with your fingers until the streusel is a mixture of larger lumps and fine sandy textured streusel. Set aside.

Make muffins

• Melt ¼ cup unsalted butter. Add ¼ cup of neutral oil, such as canola or avocado oil to cool it off. Set aside.

In a large bowl, use a fork to mash your bananas until just a few lumps remain. If you prefer them more smooth, keep going, or use a hand mixer to pulverize them. Add eggs, vanilla, and Greek yogurt and whisk together.
• Whisk together your dry ingredients and add to the wet ingredients. Mix until almost combined. Drizzle in the butter and oil mixture and stir until everything is incorporated. Gently fold in your toasted walnuts.
• Using a medium scoop or measuring cup, place ¼ cup of batter into each muffin cup, or until all the batter is used. Sprinkle each one on top with a rounded tablespoon of streusel. If you have leftover streusel, evenly divide it among the muffins.
• Bake at 425° for 5 minutes. After 5 minutes turn the oven down to 350°, without opening the oven door. Bake for another 20 to 22 minutes, or until a clean toothpick inserted into the center muffin comes out with few or no crumbs attached.