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)