Preheat the oven to 350°F. Melt the extra 2 tablespoon of butter in a microwave-safe dish. Brush the melted butter throughout the interior of the bundt pan using a pastry brush. Dust the greased interior with a light flour coating, ensuring the interior is completely greased and flour-coated. Set the cake pan aside.
Whisk the ¾ cup of light brown sugar and the tablespoon of ground cinnamon in a small bowl.
In a large mixing bowl using a hand mixer (or in the bowl of a stand mixer), beat the butter and sugar until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, beating well after each addition. Mix the vanilla extract in until combined.
Whisk the flour, cornstarch, baking powder, salt, and ground cinnamon in a large bowl. Mix half of the dry ingredients into the butter mixture until incorporated. Pour in the milk and sour cream, mixing until combined. Add in the remaining dry ingredients, again mixing just until blended.
Pour half the batter into the prepared bundt pan and smooth the top with a rubber spatula. Sprinkle the cake batter with the brown sugar-cinnamon mixture. Pour the remaining cake batter over the filling and smooth the top with a rubber spatula. Tap the cake pan on the counter 5-10 times to prevent air pockets.
Bake the cake in the preheated oven for 50-55 minutes or until a toothpick inserted into the center of the cake comes out clean.
Cool the cake for 10-15 minutes before flipping the cake pan over onto a wire rack. Cool the cake completely.