Preheat the oven to 325°F (163°C). Grease and flour a 10-cup bundt pan thoroughly, getting into every ridge.
Whisk the brown sugar, flour, and cinnamon together, then stir in the melted butter and chopped pecans to form a thick paste. Set aside.
Beat the butter and cream cheese together for about 2 minutes until smooth. Add the granulated sugar and beat for 3 more minutes.
Add the eggs one at a time, beating briefly after each. Mix in the vanilla.
Whisk the flour, baking powder, and salt together. Add to the batter in three additions, alternating with the sour cream in two additions, mixing on low just until combined.
Spread a third of the batter into the pan. Dollop half the cinnamon-pecan filling over it, leaving a border from the pan sides. Repeat with another third of batter and the remaining filling, then top with the last of the batter.
Bake 60–70 minutes, until a toothpick in the thickest part comes out with a few moist crumbs.
Cool in the pan on a wire rack for 20 minutes, then invert onto the rack and cool completely, at least 1 hour.
Warm the caramel sauce until pourable, stir in the pecans, and pour over the cooled cake.
Whisk the cream cheese, powdered sugar, milk, and vanilla until smooth. Drizzle over the caramel layer and let set 10 minutes before slicing.