In a small microwave safe bowl, melt the butter. Mix in the flour, sugar, and cinnamon until well combined. Set aside.
Coffee Cake Muffins
Preheat the oven to 400°F / 205°C. Line a muffin pan with 6 parchment liners.
In a medium sized microwave safe bowl, melt the butter. Whisk in the sugar, followed by the egg and vanilla.
Whisk in the sour cream until well combined.
Add in the flour, baking powder, baking soda, cinnamon, and salt. Fold the mixture together just until no flour streaks remain.
Evenly distribute the muffin batter between the six parchment liners. Top with the streusel.
Bake the muffins at 400°F for 5 minutes. Without opening the oven door, reduce the temperature to 350°F / 175°C. Bake for an additional 10 minutes, or until a toothpick inserted into the center of the muffins comes out clean.