Preheat oven to 350°F (175°C). Grease a 9x9-inch ceramic or metal baking pan and line with parchment paper with overhang for easy removal.

In a small bowl, mix dark brown sugar, cinnamon, flour, and melted butter with a fork until crumbly streusel forms. Reserve half for the swirl and half for the topping.

In a large bowl or stand mixer, beat softened butter and granulated sugar for 3 minutes until pale and fluffy. Add eggs one at a time, then mix in sour cream and vanilla until smooth.

Whisk flour, baking powder, baking soda, and salt together, then fold into wet ingredients with a rubber spatula just until combined (do not overmix).

Spread half the thick cake batter into the prepared pan. Scatter half the cinnamon mixture across the surface. Dollop remaining batter on top, sprinkle remaining cinnamon streusel, and use a knife to swirl in figure-eight patterns.

Bake for 38 to 42 minutes until golden-brown and a toothpick inserted into the center comes out clean with a few moist crumbs. Cool in pan for 15 minutes.

Whisk softened cream cheese, powdered sugar, vanilla, and warm milk until smooth and pourable. Drizzle lavishly over the warm cake. Slice and serve warm!