Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it lightly.
In a large bowl, combine the flour, granulated sugar, baking powder, baking soda, salt, and 1 teaspoon of cinnamon. Mix well.
In another bowl, whisk together the melted butter, eggs, vanilla extract, and sour cream until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix; a few lumps are okay.
In a separate small bowl, mix together the brown sugar, 1 teaspoon of cinnamon, and chopped nuts (if using) to create the crumble topping.
Scoop about 2 tablespoons of muffin batter into each muffin cup, then sprinkle a generous tablespoon of the crumble mixture on top. Add another layer of batter, filling each cup about ¾ full, and finish with more crumble on top.
Bake for 18-22 minutes, or until a toothpick inserted in the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.