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Preheat oven to 350℉. Grease a 13×9-inch pan with nonstick cooking spray. (I like the kind with flour in it.) Set aside.
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CAKE: In a large bowl, whisk together the flour, baking powder, baking soda, and salt. Set the dry ingredients aside.
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In the bowl of a stand mixer with the paddle attachment, cream the butter and sugar until light and fluffy. Add the eggs, vanilla, and sour cream. Mix together completely, scraping the bowl as needed.
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Add half of the dry ingredients. Mix on low until just incorporated. Add all of the milk. Mix on low until just incorporated.
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Add the remaining dry ingredients. Mix on low until just incorporated. Do not over-mix. The batter will be thick, but smooth.
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CINNAMON FILLING: Melt the butter in a microwave-safe bowl in the microwave. Add the brown sugar, cinnamon, and flour. Stir together completely. Mixture should be thin and runny. NOTE: Do not let the filling cool and thicken, or it will be hard to swirl with the cake batter.
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Spread the cake batter into the prepared 13×9-inch pan. Spoon the cinnamon-sugar mixture over the cake, including close to the edges, leaving some cake batter showing. Use the tip of a butter knife to swirl the two mixtures together.
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Bake the cinnamon roll cake at 350℉ for 25-30 minutes, or until a toothpick inserted into the center of the cake comes out clean or with a few moist crumbs. Allow the cake to cool for 15-20 minutes before adding glaze.
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GLAZE: In a small bowl, stir together the powdered sugar, vanilla and milk. Drizzle or pour glaze over the cake. Serve while still slightly warm.