1/2cup(113g/4 ounces) unsalted butter, at room temperature
1cup(198g/7 ounces) granulated sugar
1/4cup(53g/1 and 7/8 ounces) light brown sugar, packed
2large eggs plus 1 egg yolk, at room temperature
1and 1/2 teaspoons vanilla extract
1/2cup(113g/4 ounces) buttermilk
1/2cup(113g/4 ounces) full-fat sour cream
For the Glaze:
1cup(113g/4 ounces) confectioners' sugar, sifted
2 to 3Tablespoonsbuttermilk
For the Buttery Crumb Topping:
In a medium mixing bowl, combine the flour, sugar, cinnamon, and salt. Stir in the melted butter, mixing just until combined. Set aside while you prepare the rest of the recipe. The mixture will absorb the butter as it sits and become more crumbly by the time you’re ready to use it.
For the Cinnamon Sugar Layer:
In a small mixing bowl, combine the sugar and cinnamon. Set aside.
For the Muffins:
Preheat oven to 400°F (204°C). Line a 12 cup muffin tin with paper liners and set it aside.
In a large mixing bowl, combine the flour, baking powder, cinnamon, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a handheld electric mixer, combine the butter and both sugars and beat on medium-low speed until light and fluffy; about 3 minutes. Add in the eggs and yolk, one at a time, beating well after each addition. Slowly beat in the vanilla, buttermilk, and sour cream. Reduce mixer speed to low and gradually add in the dry ingredients, mixing just until combined. Be sure not to over mix here.
For the Glaze:
In a small mixing bowl, whisk together the confectioners' sugar, buttermilk, vanilla, and salt until smooth. Add more confectioners' sugar to thicken or more buttermilk to thin.
Scoop a large spoonful of batter into each prepared muffin cup and, using a lightly greased spoon, spread it into the edges of the cup. Evenly divide the cinnamon sugar on top of the batter, aiming to pile it in the center of the each cup. Divide remaining batter evenly among the muffin cups, then use a lightly greased spoon to push it down on the sides to enclose the cinnamon sugar.
Crumble up the topping and evenly distribute it on top of the 12 muffins, piling it high in the center. Gently push the streusel into the batter. This helps it stay put as the muffins bake.
Bake for 20 to 22 minutes, or until the tops are lightly golden brown and a toothpick inserted in the middle comes out clean. Allow the muffins to cool in the pan, placed on top of a wire rack, for 10 minutes, then transfer them to a large serving plate.
Drizzle a little glaze on top of each muffin and serve warm.
To store, cool completely, then wrap muffins tightly in plastic wrap, and keep at room temperature, for up to 2 days.