Preheat oven to 425°F (218°C). Spray a 12-count muffin pan with nonstick spray or line with cupcake liners. Set aside.

In a medium bowl, mix the brown sugar, granulated sugar, and cinnamon. Stir in the melted butter. Using a fork, gently work in the flour just enough to combine. Do not overmix; the mixture should have large crumbles. Set topping aside or refrigerate until ready to use.

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

In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter, granulated sugar, and brown sugar on high speed until smooth and creamy, about 3 minutes. Scrape down the sides and bottom of the bowl as needed. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the sour cream, eggs, and vanilla. Beat on medium speed for 1 minute, then turn up to high speed until the mixture is combined. Scrape down the sides and bottom of the bowl as needed.

Pour the dry ingredients into the wet ingredients and beat on low speed until just about combined. Add the milk and continue to beat on low speed until combined. Fold in your chosen add-ins. (See flavors/details in the Notes below.) 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 to the batter.

Bake the muffins for 5 minutes at 425°F (218°C); then, keeping the muffins in the oven, reduce the oven temperature to 350°F (177°C). Bake for an additional 15–18 minutes or until a toothpick inserted in the center comes out clean. The total time these muffins take in the oven is about 22–23 minutes, give or take. Allow the muffins to cool for 10 minutes in the muffin pan set on a cooling rack, then transfer the muffins from the pan to the rack to cool completely.

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