Heat the oven to 325 degrees F and coat an 18-cup tube pan thoroughly with nonstick baking spray. Make sure the spray reaches every curve and crease so the cake can release cleanly later.

Make the crumb topping by mixing the flour, sugar, and cinnamon in a medium bowl, then work in the room temperature butter and shortening with your fingers or a fork until crumbles form. The mixture should look sandy with small clumps, not like a smooth paste, then chill it in the refrigerator.

Stir the brown sugar and cinnamon together in a small bowl for the filling and set it aside. You want the cinnamon evenly distributed so the ribbon tastes balanced from edge to center.

Whisk the flour, cinnamon, baking powder, and kosher salt in a separate bowl. This helps the leavening and spice disperse evenly before they meet the wet ingredients.

Cream the butter, shortening, sugar, and brown sugar in a stand mixer with the paddle attachment for 5 minutes, then add the eggs one at a time on low speed, scraping the bowl between additions. After the full creaming time, the mixture should look paler, noticeably fluffier, and a little billowy rather than dense and grainy.

Mix in half of the dry ingredients for about 20 seconds, add the milk, sour cream, and vanilla extract and mix for about 10-15 seconds, then scrape the bowl and add the remaining dry ingredients, mixing about 10 seconds more until smooth. Stop as soon as the batter loses dry streaks and looks thick and creamy, because too much mixing at this stage can make the crumb dense.

Scrape the sides and bottom of the bowl one last time to be sure the batter is evenly combined. The finished batter should look uniform and spreadable, with no pockets of butter or flour hiding underneath.

Add 1/3 of the batter to the prepared pan, sprinkle over half of the cinnamon filling, add another 1/3 of the batter, top with the remaining filling, and finish with the last 1/3 of the batter. Cover the filling fully with the top layer so the cinnamon stays suspended in a ribbon instead of baking exposed on the surface.

Scatter the chilled crumb topping evenly over the batter. Try to cover the surface from edge to center so each slice gets a bit of crisp, buttery top.

Bake for 1 hour and 20-30 minutes until a toothpick comes out with a few crumbs, then let the cake cool completely in the pan. The cake should look set across the top and smell deeply of butter, cinnamon, and brown sugar.

Melt the butter in a skillet over medium heat and keep stirring until it foams and browned bits gather on the bottom, about 10 minutes. When it reaches peak nuttiness, the butter smells toasted and warm, almost like hazelnuts, and the milk solids turn amber rather than black.

Transfer the browned butter to a mixing bowl and whisk in the powdered sugar, milk, and vanilla, adding more milk 1 tbsp at a time until the glaze reaches the thickness you want. A good drizzle should fall in ribbons, not splash like milk or sit in a stiff lump on the whisk.

Pour the glaze over the cooled cake. Let it run naturally over the curves so you get clean lines and little pockets of glaze in the ridges.