Preheat oven to 350°F. Grease a bundt cake pan or a tube pan well with cooking spray.
Place the cake mix, sugar, oil, vanilla, sour cream, and eggs in the bowl of a stand mixer. Mix gently until combined.
In a small bowl, combine the remaining pecans, brown sugar, and cinnamon.
Pour half of the batter into the greased pan. Sprinkle the pecan mixture over the top, taking care to keep the mixture in the center of the batter (don't let it touch the sides). Pour the remaining batter over the pecans.
Bake for 45–50 minutes, or until a toothpick comes out clean.
Let the cake cool completely, inverting it onto a cooling rack after 20 minutes cooling in the pan.
Once cool, whisk together the powdered sugar and just enough milk to make a pourable consistency. Drizzle the icing over the cake, let harden, and serve.
Notes
Take care to sprinkle the pecan mixture in the center of the batter. If it touches the sides, it's likely to stick to the pan when you invert the cake. You can make the cake up to 2 days in advance. Cover it tightly and leave it at room temperature until you're ready to serve it. You can freeze the cake (either whole or in slices) for up to 2 months. Wrap it tightly in plastic wrap then aluminum foil before freezing. Instructions for a 13x9 inch pan: Grease the pan, then pour in half the batter, top with the pecan mixture, and top with the remaining batter. Bake at 350°F for 30–35 minutes.