Preheat oven to 350°F. Generously spray your Bundt pan, getting into all the ridges.

Stir the brown sugar, flour, cinnamon, and salt together. Pour in the melted butter and stir until it clumps like wet sand. Set aside.

In a large bowl, mix the cake mix, pudding, sour cream, melted butter, milk, eggs, cinnamon, vanilla, and cream of tartar 30 seconds on low, 2 minutes on medium.

Spoon half the batter into the pan and smooth it out.

Crumble the streusel down the center of the batter, keeping it off the sides so it bakes into a ribbon instead of leaking out.

Spoon the remaining batter over the top and smooth to the edges to seal the streusel inside.

Bake 45 to 55 minutes, until a skewer comes out clean.

Cool in the pan 5 minutes, then invert onto a rack.

While the cake is still warm, brush the entire outside with the melted butter.

Press the cinnamon sugar into every ridge with your hands. You will likely have some left over, and that is okay.