How to Make It
Make the streusel topping: In a medium bowl, mix together the flour, sugars, cinnamon and salt. Using your hands, work in the cubed butter until pea-sized crumbles form. Set aside.
Make the muffin: Preheat the oven to 325° and grease a 9-inch springform pan with butter. Create a ring of foil to place flush with the top of the pan.
In a medium bowl, whisk together the flour, baking powder and salt. In the bowl of stand mixer fitted with the paddle attachment, cream the stick of butter with the sugar until light and fluffy.
With the motor running on low, add the eggs, one at a time until incorporated, followed by the dry ingredients and the buttermilk until a smooth batter forms. Pour into the prepared pan and top with the crumb topping. Bake until golden and a toothpick inserted in the center comes out clean, 2 hours to 2 hours and 15 minutes. Let cool completely. Invert onto a plate, remove the springform pan and invert back onto a platter.
Make the glaze: In a small bowl, whisk the glaze ingredients until smooth. Drizzle over the muffin, then slice and serve.