Prep the pan and oven. Heat the oven to 350°F (175°C). Grease and line an 8-inch square pan or a 9-inch round pan with parchment, leaving some overhang for easy lifting.

Make the cinnamon mixture. In a small bowl, combine brown sugar, cinnamon, flour, and a pinch of salt. If using the butter, cut it in with your fingers until sandy. Set aside.

Whisk the wet ingredients. In a large bowl, whisk melted butter, granulated sugar, and brown sugar until glossy. Whisk in eggs one at a time until smooth. Add sour cream and vanilla; whisk until combined.

Combine the dry ingredients. In a separate bowl, mix flour, baking powder, baking soda, and salt.

Bring the batter together. Add the dry ingredients to the wet. Stir gently with a spatula just until no dry streaks remain. Do not overmix.

Layer the cake. Spread half the batter in the pan. Sprinkle half the cinnamon mixture over it. Dollop the remaining batter on top and spread gently. Finish with the rest of the cinnamon mixture.

Swirl (optional). For a marbled look, run a butter knife through the batter in a few gentle S-shapes.

Bake. Bake for 35–45 minutes, until the top springs back and a toothpick comes out clean or with a few moist crumbs. If the top browns too quickly, tent loosely with foil near the end.

Cool. Let the cake cool in the pan for 15 minutes, then lift it out using the parchment and transfer to a rack to cool to barely warm.

Glaze. Whisk powdered sugar, milk, vanilla, and a pinch of salt until smooth and pourable. Drizzle over the warm cake. Let it set for 10 minutes, then slice and serve.