Step 1: Prepare the Batter

1. Preheat oven to 350°F (175°C) and grease a 9×13-inch baking pan or bundt pan.
2. In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
3. In a large bowl, beat the butter and sugar together until light and fluffy (about 2-3 minutes).
4. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
5. Alternate adding the dry ingredients and sour cream, mixing just until combined.

Step 2: Layer the Cinnamon Swirl

1. In a small bowl, mix brown sugar and cinnamon.
2. Pour half of the batter into the prepared pan and spread evenly.
3. Sprinkle half of the cinnamon sugar mixture over the batter.
4. Pour the remaining batter on top and smooth it out.
5. Finish by sprinkling the remaining cinnamon sugar over the top.

Step 3: Bake to Perfection

1. Bake for 35-40 minutes (for a 9×13 pan) or 45-50 minutes (for a bundt pan) until a toothpick inserted in the center comes out clean.
2. Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

Step 4: Make the Sour Cream Glaze

1. In a small bowl, whisk together powdered sugar, sour cream, vanilla, and milk until smooth.
2. Drizzle over the cooled coffee cake.

Step 5: Slice & Enjoy!

Cut yourself a thick slice, pour a hot cup of coffee, and enjoy every buttery, cinnamon-infused bite.