01 -
Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking pan well with butter or nonstick spray.
02 -
In a large bowl, whisk flour, sugar, baking powder, baking soda, and salt. Add sour cream, softened butter, eggs, and vanilla. Mix until thick and smooth with no dry streaks.
03 -
Scrape the batter into the prepared pan and spread it evenly into the corners with a spatula.
04 -
In a medium bowl, stir brown sugar, very soft butter, flour, and cinnamon until it forms a thick spreadable paste.
05 -
Drop spoonfuls of the cinnamon mixture over the batter. Use a butter knife to swirl through the top half of the batter in wide loops, taking care not to overmix.
06 -
Bake for 32-38 minutes, until the top springs back lightly and a toothpick inserted near the center comes out with moist crumbs.
07 -
Cool the cake for 8-10 minutes. Whisk powdered sugar, 4 tablespoons milk, vanilla, and salt until smooth, adding more milk if needed. Pour over the warm cake and let it settle before slicing.