Preheat the oven to 325 degrees Fahrenheit. Prepare a bundt pan by using a paper towel to coat the inside with melted shortening and a light dusting of flour (or granulated sugar) over that, tapping out the excess. You may alternatively use a baking spray that has flour in it.
Sift together 3 cups all-purpose flour, ½ teaspoon baking soda, and ½ teaspoon salt in a medium bowl and set aside.
In a stand mixer, beat 3 cups granulated sugar and 1 cup unsalted butter until light and fluffy, scraping down the sides and bottom of the bowl as needed.
Add 6 large eggs one at a time and beat well after each addition, followed by 1 cup sour cream and 1 tablespoon vanilla extract. Scrape down the bottom and sides of the bowl.
On low speed, gradually add in the flour mixture until fully combined.
Combine 1 cup chopped pecans, ½ cup packed brown sugar, and 2 teaspoons ground cinnamon in a small bowl.
Pour half of the batter into the prepared bundt pan and spread it out evenly.
Sprinkle the pecan mixture evenly over the batter before topping with the remaining batter and smooth it over the top.
Bake for about 80 minutes, or until a toothpick comes out nearly completely clean from the center of the cake, with only a few crumbs clinging to it. Let the cake cool in the pan for only 5 minutes before inverting it onto a cooling rack and removing the pan to let it cool completely.
Once the cake has cooled, make the glaze by whisking 1 cup powdered sugar and 3 tablespoons milk together in a small bowl until smooth. Drizzle over the top of the cake.