In a stand mixer fitted with a paddle attachment, cream the butter and shortening on speed 3 (for a 10 speed mixer).
Next, add the sugar gradually while the mixer is still going, or until light and fluffy. Note that the mixer should be at speed 3 or 4 and no higher. If too much air mixes in, the cake can collapse during baking.
With the mixer still running, add the vanilla and sour cream, and then the eggs one at a time. As soon as the eggs are mixed in, stop mixing! Over mixing the eggs will lead to a tougher and more crunchy texture.
Sift the cake flour and combine with the salt. With the mixer on low, add the flour and evaporated milk alternately in 2 additions each. Stop mixing when there is still some flour showing.
Remove the batter from the stand mixer and gently stir in the remaining flour. The batter may not be perfectly smooth, but that’s okay.
Next, mix the cinnamon ingredients in a bowl, and then begin assembling the cake batter and cinnamon filling in the bundt cake pan.
Use a muffin scoop or large cookie scoop to portion 4-5 heaping scoops into the bottom of the pan followed by a layer of the cinnamon pecan mixture. Top with another 6-7 heaping scoops, smooth out, and then more cinnamon mixture. Repeat the same process until the batter is gone. You should end with batter on top, even if it’s just a thin amount. This will lock the cinnamon in place so that it doesn’t spill out when flipping the cake out of the pan.
Place cake in a cold oven and turn the temperature to 325 degrees. Bake for 70 minutes, then turn the temperature to 350 and bake for another 15-20 minutes. Even if the cinnamon swirl bundt cake looks golden brown on top, check the inside with a knife or skewer. The inside of the cake may still be underdone regardless of the outer appearance. I find that it almost always needs to bake the full 90 minutes.
Remove cake from the oven and let cool completely before removing from the pan.
For the Cinnamon Glaze
Whisk the powdered sugar and milk together. Start with just 3 tablespoons of milk, and add more until the icing can drizzle off of the whisk in a slow ribbon. If it’s a quick ribbon, the icing is too thin.
Pour or drizzle over the cooled cake, and top with chopped pecans. Cut, serve, and enjoy! This cake can be stored in an airtight container for up to a week at room temperature.