Preheat the oven to 425°F.

Line a 12-cavity muffin pan with muffin/cupcake liners.

In large mixing bowl, use a hand mixer to cream together the butter, granulated sugar, brown sugar, and vanilla extract on medium-high speed until smooth and combined, 1-2 minutes.

Add the eggs and beat for a minute longer, until the mixture is pale and fluffy. Add the sour cream and milk. Continue to beat until fully incorporated, scraping down the sides of the bowl with a rubber spatula.

Add the flour, baking powder, baking soda, cinnamon, and salt to the bowl. Beat the mixture on medium-low speed, scraping down the sides of the bowl with a rubber spatula as needed, until just combined. Do not over-mix.

To make the streusel, place the flour, brown sugar, granulated sugar, cinnamon, and salt in a large mixing bowl. Stir with a fork to combine. Drizzle the melted butter into the bowl, and mix with the fork until the mixture forms large crumbs.

Fill each muffin liner approximately 1/3 full with muffin batter. Crumble a scant tablespoon of streusel into each muffin cup. Divide the remaining batter into the muffin cups. Sprinkle the muffins with the remaining streusel.

Bake the muffins on middle rack of oven for 5 minutes.

Reduce the oven temperature to 350°F. Continue baking until a toothpick inserted in the center of a muffin comes out clean, 14-16 minutes.

Check to see that muffins are done. Remove from oven or add time as needed.

Allow the muffins to cool in the pan for 5 minutes before removing and serving. Store leftovers in an airtight container at room temperature for 2 days, or refrigerate for 3 days.