Preheat oven to 375°F.

Prepare a muffin pan by spraying each cup with non-stick spray or line them with cupcake liners.

Make the Topping

• Combine all dry topping ingredients in a medium bowl and mix until well blended. Next, Incorporate softened room-temperature butter with your fingers until the mixture reaches a crumbly consistency.
  ½ cup all purpose flour, ½ cup brown sugar, ¼ cup sugar (granulated), 4 tablespoons butter, ½ teaspoon ground cinnamon, ¼ teaspoon ground nutmeg

Make the Batter

• Cream Butter and Sugar. Cream together the butter and sugar on medium-high speed until light and fluffy and a pale yellow color (about 3-5 minutes).
  1 ½ cup sugar (granulated), ¾ cup butter
• Combine Wet Ingredients. In a separate bowl, combine mashed bananas, eggs, and vanilla.
  1 ½ cup bananas, 2 eggs, 1 teaspoon pure vanilla extract
• Combine Creamed Butter and Sugar Mixture With Wet Ingredients. Add the banana mixture to the creamed butter and sugar and mix until well combined.
• Sift Together Dry Ingredients. In a separate bowl, sift together the flour, baking soda, and salt.
  2 cups all purpose flour, ¾ teaspoon salt, 1 teaspoon baking soda
• Mix Batter. Add the dry ingredients and sour cream to the wet ingredients mixture, a little at a time, alternating between the two.

  Mix until just combined, being careful not to overmix. The batter should be thick, and smooth, with little pieces of banana throughout.
  ½ cup sour cream
• Fill the Muffin Cups. Fill each cup ¾ full with batter.
• Sprinkle Topping. Sprinkle topping over the batter in each cup.
• Bake. Bake the muffins in a [metal or silicone] muffin tin on the center rack at 375°F (190°C) for 28-32 minutes, or until a toothpick inserted into the center comes out clean.
• Cool. Let the muffins cool in the pan for 5-10 minutes before transferring them to a wire rack to cool completely.