Preheat your oven to 425 degrees Fahrenheit. Spray a 12-count muffin pan with nonstick spray or line it with paper liners. Set aside.

In a medium bowl, combine the brown sugar, granulated sugar, and cinnamon. Stir in the melted butter. Using a fork, gently work in the flour just until combined. The mixture should be crumbly and not overmixed. Set aside or refrigerate until ready to use.

In a large bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon (if using). Set aside.

In another large bowl, using a hand mixer or stand mixer fitted with a paddle attachment, beat the softened butter, granulated sugar, and brown sugar on high speed until smooth and creamy, about 3 minutes. Scrape down the sides and bottom as needed. Add the sour cream, eggs, and vanilla. Beat on medium speed for 1 minute, then increase to high speed until combined. Scrape down again.

Pour the dry ingredients into the wet mixture and beat on low speed until just barely combined. Add the milk and beat on low until combined. Fold in your chosen add-ins. The batter should be thick.

Spoon the batter into the prepared muffin cups, filling them all the way to the top. Sprinkle the tops with coarse sugar or spoon on the crumb topping and lightly press the crumbles so they stick.

Bake the muffins for 5 minutes at 425 degrees Fahrenheit. Then, keeping the muffins in the oven, reduce the temperature to 350 degrees Fahrenheit and bake for an additional 15 to 18 minutes, or until a toothpick inserted in the center comes out clean. Total baking time is about 22 to 23 minutes. Let the muffins cool in the pan on a wire rack for 10 minutes, then transfer them directly to the rack to cool completely.

Store muffins tightly covered at room temperature for up to 5 days, or in the refrigerator for up to 1 week.