Step 1: Prepare the Cinnamon Filling. In a bowl, combine 2 3/4 cups of all-purpose flour, 1 1/2 cups of brown sugar, 1 teaspoon of kosher salt, and 2 tablespoons of ground cinnamon. Mix well and set aside.
Step 2: Make the Cake Batter. In a separate bowl, whisk together 3 cups of all-purpose flour, 2 teaspoons of baking powder, 3/4 teaspoon of baking soda, and 1 teaspoon of kosher salt.
Step 3: Cream the Butter and Sugar. In a large mixing bowl, combine 1 cup of unsalted butter (at room temperature) and 1 1/2 cups of granulated sugar. Using a stand mixer or hand mixer, cream them together until the mixture is light and fluffy.
Step 4: Add the Eggs and Sour Cream. Add 4 large eggs (one at a time) to the butter-sugar mixture, mixing well after each addition. Then, add 1 1/2 cups of sour cream and 1 tablespoon of vanilla extract. Mix until everything is well combined.
Step 5: Combine Dry Ingredients. Gradually add the dry flour mixture to the wet ingredients, mixing gently until just combined. Be careful not to overmix!
Step 6: Assemble the Cake. Preheat your oven to 350°F (175°C). Grease your 9x13 inch baking pan. Pour half of the batter into the prepared pan and smooth it out with a spatula. Sprinkle the cinnamon filling mixture evenly over the batter, then pour the remaining batter on top, smoothing it out again.
Step 7: Bake. Place the pan in the preheated oven and bake for 45-50 minutes or until a toothpick inserted into the center comes out clean.
Step 8: Cool and Glaze. Once baked, remove the cake from the oven and let it cool in the pan for about 10 minutes. Then, transfer it to a cooling rack to cool completely. For a finishing touch, whisk together 1 cup of confectioners sugar, 2 tablespoons of milk or heavy cream, and 1/4 teaspoon of pure vanilla extract. Drizzle this glaze over the cooled cake for a sweet finish.