Preheat your oven to 350°F (175°C). Grease a 10- to 12-cup Bundt pan thoroughly with butter or baking spray, making sure to coat all the grooves. Lightly dust the pan with flour and tap out any excess to help prevent sticking.
In a medium bowl, combine the brown sugar, ground cinnamon, chopped pecans, flour, and melted butter. Stir until the ingredients are evenly mixed and form a crumbly filling. Set aside while you prepare the cake batter.
In a large mixing bowl, beat the softened butter and granulated sugar with an electric mixer for approximately 3–5 minutes, until the mixture looks pale and fluffy. This step incorporates air into the batter and helps create a tender cake.
Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract until evenly combined. Scrape the sides of the bowl as needed to ensure the ingredients are thoroughly incorporated.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Add the dry mixture to the butter mixture in batches, alternating with the sour cream. Begin and finish with the dry ingredients, mixing on low speed just until combined. Add a little milk if the batter is unusually thick.
Spread approximately half of the cake batter into the prepared Bundt pan. Sprinkle the cinnamon-pecan filling evenly over the batter, keeping it away from the outer and inner edges of the pan where possible. Carefully spoon the remaining batter over the filling and smooth the surface.
Place the Bundt pan on the center rack of the preheated oven. Bake for 50–60 minutes, or until a wooden skewer inserted into the cake portion comes out clean or with a few moist crumbs. Avoid inserting the skewer directly into the cinnamon filling when testing for doneness.
Remove the cake from the oven and let it rest in the pan for 15–20 minutes. Carefully loosen the edges if necessary, then invert it onto a wire rack. Allow the cake to cool completely before adding the glaze.
Whisk the powdered sugar, milk, and vanilla extract in a small bowl until smooth. Add a little more milk for a thinner glaze or additional powdered sugar for a thicker consistency.
Drizzle the glaze over the cooled cake, allowing it to run naturally down the sides. Alternatively, dust the cake with powdered sugar. Slice and serve with coffee, tea, or your favorite beverage.