• 1/3 cup sugar (2 2/3 ounces)
• 1/3 cup dark brown sugar (2 2/3 ounces)
• 3/4 teaspoon ground cinnamon

• 1/8 teaspoon salt
• 8 tablespoons unsalted butter, melted but warm
• 1 3/4 cup cake flour (7 ounces)

• 1 1/4 cup cake flour (5 ounces)
• 1/2 cup sugar (3 1/2 ounces)
• 1/4 teaspoon baking soda
• 1/4 teaspoon salt
• 6 tablespoons unsalted butter, cut into 6 pieces, softened but still cool

• 1 large egg
• 1 large egg yolk
• 1 teaspoon vanilla extract
• 1/3 cup buttermilk

For the topping, whisk sugars, cinnamon, salt and butter in a medium bowl. Add flour and stir with a rubber spatula or wooden spoon until it comes together as a dough. Set aside at room temperature to cool.

For the muffins, heat oven to 325° and position the rack in the upper-middle part of the oven. Line standard sized muffin tins with baking liners. Mix flour, sugar, baking soda and salt on low speed to combine. Add butter one piece at a time and beat until mixture resembles moist crumbs and butter has been incorporated, about 2 minutes. Add egg, yolk, vanilla and buttermilk. Beat on medium-high speed until light and fluffy, about 1 minute.

Use a 1/4 cup measuring cup or scoop to divide batter evenly among muffin cups and even out the tops with a spatula. Break the crumb mixture into large pea sized pieces and spread in an even layer over the smoothed tops of each muffin to cover. Bake about 20 minutes or until a toothpick comes out clean and crumbs are golden. Transfer to a wire rack and cool at least 20 minutes.