For the crumb topping:
1/3 cup granulated sugar
1/3 cup dark brown sugar
3/4 tsp. ground cinnamon
1/8 tsp. salt
8 tbsp. unsalted butter, melted
1 3/4 cups cake flour
For the muffin:
1 1/4 cup cake flour
1/2 cup granulated sugar
1/4 tsp. baking soda
1/4 tsp. salt
6 tbsp. unsalted butter, cut into 6 pieces, softened but still cool
1 large egg
1 egg yolk
1 tsp. vanilla extract
1/3 cup buttermilk (or 1/3 cup plain yogurt)
1. Preheat oven to 325 degrees. Line muffin pan with baking cups or prepare panettone baking cups by spraying them lightly with cooking spray.
2. In a medium bowl, whisk together sugars, cinnamon, and salt. Pour the butter over top and using a wooden spoon, mix until incorporated. Then add the cake flour and mix until mixture resembles a thick and cohesive dough. Set aside.
3. In the bowl of a stand mixer fitted with the paddle attachment, mix flour, sugar, baking soda, and salt on low speed to combine. With mixer at low speed, add butter one piece at a time; continue beating until mixture resembles moist crumbs, with no visible butter chunks remaining, 1 to 2 minutes.
4. Add the egg, egg yolk, vanilla extract and buttermilk and mix until all has been incorporated and batter is light and fluffy.
5. Using a 1/4 cup measuring cup, divide up batter into prepared muffin pan. Then, sprinkle a generous amount of the crumb topping on top. Do not press the crumb topping into the batter. Just let it sit on top.
6. Bake for 20 minutes or until crumbs are golden and a toothpick inserted into the middle comes out clean. (Note: I did stick my muffins under the broiler at about the 15 minute baking mark so the tops would get nice and brown. If you do this, make sure to watch it carefully. You do not have to do the broiler step and can omit it completely.)
7. Let muffins cool for 5 minutes before transferring them to a wire rack to cool completely.
8. Store in an airtight container for up to 2 days.