Preheat the oven to 350°F (175°C). Grease and flour a 10-cup bundt or tube pan generously, making sure to get into all the crevices.

In a small bowl, mix together the brown sugar, cinnamon, melted butter, and flour until combined. Set aside.

In a large bowl, cream together the butter and sugar until light and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking soda, baking powder, salt, and cinnamon.

In another small bowl or measuring cup, whisk together the sour cream and milk until smooth.

Add the dry ingredients to the creamed mixture in three additions, alternating with the sour cream and milk mixture, beginning and ending with the dry ingredients. Stir in the dry instant pudding mix and mix until just combined. Do not overmix.

Pour one-third of the batter into the prepared pan. Spoon one-third of the cinnamon swirl mixture evenly over the batter. Repeat two more times, ending with the swirl mixture on top. Use a knife or skewer to lightly swirl the layers together—just a couple of figure-eights is enough.

Bake for 50–60 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs. If the top starts to brown too quickly, tent it with foil around the 45-minute mark.

Let the cake cool in the pan for 15–20 minutes before carefully inverting it onto a wire rack or plate. Do not let it sit in the pan too long, or the swirl may stick.

Once fully cool, dust with powdered sugar or drizzle with glaze, if desired. Slice and enjoy!