In a medium bowl, whisk together the flour, sugar, salt, and cinnamon until combined. Add melted butter and vanilla extract to the flour mixture. Stir together with a fork until the topping is pea-sized crumbs.
Make the batter
Preheat oven to 350ºF.
Add the butter and sugar to a large mixing bowl (or stand mixer bowl). Use a mixer on high speed to cream the butter and sugar together. Add eggs to the batter, one at a time, mixing for 15 seconds each. Add vanilla extract and sour cream, and beat on medium speed until the mixture is smooth. As necessary, scrape down the insides of the bowl with a silicone spatula to be sure the batter is well combined.
In a medium bowl, whisk together flour, cinnamon, baking powder, baking soda, and salt. Add to the creamed butter mixture and stir until the batter is smooth.
Spray the muffin pan with non-stick cooking spray, or use paper liners. Fill each muffin cup about 3/4 full with batter (about 1/4 cup of batter). Spoon 2 tablespoons of streusel topping evenly over each cup, to cover the muffin batter.
Bake at 350ºF for 15-20 minutes, until the coffee cake muffins are golden on top and the streusel topping is light brown. Test for doneness by inserting a wooden toothpick into the center of one muffin. Muffins are done when the toothpick comes out clean, or with a few moist crumbs.
Remove the pan of coffee cake muffins from oven and set it on a wire rack to cool for 5 minutes. Then, remove muffins from the pan and let them cool completely on the rack.
Dust the tops of the cool muffins with confectioners sugar before serving, if desired.
Notes
Sour cream substitute: 1 cup plain whole milk yogurt or plain Greek yogurt.
For tender, cake-like muffins, substitute half of the all-purpose flour in the muffins with cake flour. **Do not substitute cake flour for all-purpose flour for the cinnamon streusel topping.