Preheat oven to 350°F and grease an 8-inch square baking pan.
Make the crust by combining graham cracker crumbs, 1/4 cup brown sugar, melted butter, and salt. Press firmly into prepared pan and bake 8 minutes. Cool completely.
Reduce oven to 325°F. In blender, combine cream cheese, granulated sugar, and vanilla. Blend until smooth.
Add sour cream to blender and blend once more. Add eggs one at a time, blending between each addition until smooth.
Make cinnamon swirl by stirring together 6 tablespoons melted butter, dark brown sugar, cinnamon, and flour until smooth.
Pour half the filling over cooled crust. Dot with half the cinnamon mixture and swirl gently with knife.
Add remaining filling and cinnamon mixture. Swirl once more for marbled effect.
Bake 35-40 minutes until center is mostly set but still slightly jiggly.
Chill 2 hours until firm before slicing into bars.