-
Preheat the oven to 350°F [180°C]. Line a standard muffin tin with 12 paper liners(or a jumbo muffin tin with 6 paper liners). Alternatively thoroughly coat with nonstick cooking spray.
-
To make the streusel: combine the butter, flour, and brown sugar in a medium bowl. Use a pastry blender, a fork or rub between your fingers to break up the butter into pea-size pieces. The streusel should be a mixture of “wet sand” crumbs and pea-size chunks. Pun in the fridge while making the muffin batter. You can make it in advance and store it refrigerated in a container with a lid for up to 2 weeks.
-
To make the muffins: whisk together the melted butter (cool a little bit so it doesn’t cook the egg whites) and the eggs in a large bowl. Add the sour cream, vanilla, and mashed bananas and stir until combined. In a separate bowl, whisk together the flour, baking powder, salt and sugar.
-
Add the dry mixture to the wet mixture and carefully fold together with a spatula until just combined. Don’t overmix! Batter can still have some little flour streaks. Very gently fold in the nuts and chocolate chips. If you overmix the batter after adding the flour, the muffins will turn out tough.
-
Divide the batter equally among the prepared muffin cups, filling each about ⅔ full. You can use a cookie scoop for it. Add an equal amount of streusel on top of each muffin.
-
Bake until the toothpick comes out clean, the muffins are golden brown, 22 to 26 minutes for standard muffins or 30 to 35 minutes for jumbo.
-
Remove from the oven and let the muffins cool in the pan for 5 to 10 minutes. Carefully take out from the pan and let cool a little bit more on a wire rack. Serve warm.