Prep the pan and oven: Heat the oven to 350°F (175°C). Grease a 9x9-inch pan and line with parchment, leaving a slight overhang for easy lifting. This helps prevent sticking and keeps edges tender.

Mix the dry ingredients: In a large bowl, whisk flour, baking powder, baking soda, and salt until evenly combined. Set aside.

Combine the wet ingredients: In another bowl, whisk melted butter, sugar, eggs, vanilla, sour cream, and milk until smooth. The mixture should look creamy and glossy.

Make the batter: Pour the wet ingredients into the dry. Stir gently with a spatula until just combined. A few small lumps are fine. Do not overmix or the cake can turn dense.

Make the cinnamon swirl: In a small bowl, stir brown sugar, cinnamon, salt, melted butter, and cornstarch (if using) until it looks like wet sand.

Layer the cake: Spread half the batter in the pan. Sprinkle or drizzle half the cinnamon mixture over it. Add the remaining batter and gently spread. Top with the remaining cinnamon mixture, then use a knife to swirl in a loose, figure-eight motion. Don’t over-swirl—you want ribbons, not fully mixed batter.

Bake: Bake 32–40 minutes, depending on pan and oven. Start checking at 30 minutes. A toothpick inserted in the center should come out with a few moist crumbs but no wet batter.

Cool slightly: Let the cake cool in the pan for 15–20 minutes. This helps it set and makes slicing easier.

Glaze: Whisk powdered sugar, milk or cream, vanilla, and a pinch of salt until smooth and pourable. Drizzle over the warm cake. For thicker lines, let the cake cool longer before glazing.

Slice and serve: Cut into squares and serve warm or at room temperature. If you like, add a small pat of butter on a warm slice for extra richness.