Preheat the oven to 350ºF. Prepare 18 muffin tin slots with paper liners.

In a small bowl, combine the 2/3 cup sugar and cinnamon for the topping. Set aside.

In a medium bowl, whisk together the dry ingredients.

In a large bowl, cream together the butter and sugar with an electric mixer until fluffy, 1 to 2 minutes. Add the eggs and vanilla and combine.

Add in half of the dry ingredients to the wet ingredients and beat until combined. Add in the sour cream and buttermilk and beat to combine. Add the rest of the dry ingredients and mix until you have a smooth batter.

Using a large cookie dough scoop, plop a scoop of batter (about 3 tablespoons) into the bowl with the sugar/cinnamon. Gently roll the batter in the mixture. Place the ball into the muffin liner. (They should be 3/4 of the way full.)

When you have completed all of the muffins, sprinkle the leftover cinnamon/sugar onto the tops of all of the muffins.

Bake the muffins for about 18-20 minutes or until they are golden on top and just baked through.

Let cool for 15 minutes before serving and enjoy!