Prep the pan and oven: Heat oven to 350°F (175°C). Grease a 9x13-inch baking pan or line with parchment for easy lift-out.
Mix dry ingredients: In a large bowl, whisk flour, sugar, baking powder, and salt until combined.
Whisk wet ingredients: In another bowl, whisk milk, eggs, melted butter, sour cream, and vanilla until smooth.
Combine batter: Pour wet into dry and stir just until no dry streaks remain.
Don’t overmix. Spread batter evenly in the pan.
Make the swirl: Stir melted butter, brown sugar, cinnamon, and salt until glossy. Dollop over the batter in spoonfuls.
Create the pattern: Use a butter knife to gently swirl the cinnamon mixture through the batter, making loose figure-eights.
Bake: Bake 28–34 minutes, until the center springs back and a toothpick comes out with a few moist crumbs.
Do not overbake.
Make the glaze: Beat cream cheese until smooth. Add powdered sugar, vanilla, salt, and 2 tbsp milk. Mix until pourable; add more milk if needed.
Glaze while warm: Let the cake cool 10–15 minutes, then pour the glaze over the warm surface.
Spread to the edges and let it sink in.
Serve: Slice into squares and enjoy warm or at room temperature.