Preheat your oven to 350°F (175°C) and line a sheet pan with parchment paper to ensure easy removal after baking.
In a large bowl, combine sugar and unsalted butter, creaming them together until light and airy for about two minutes.
Add in sour cream, milk, eggs, and vanilla extract to the creamed mixture, stirring until everything is fully combined.
In another bowl, whisk together flour, baking powder, and salt so that they are evenly distributed before adding them to your wet mixture.
In yet another bowl, combine brown sugar, ground cinnamon, and melted butter; this mixture will create delightful swirls when layered over your batter.
Pour half of your batter into the prepared sheet pan evenly; drizzle half of your filling mixture on top; then add remaining batter followed by swirling in leftover filling before baking for about 23-25 minutes until golden brown on top!