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Prep. Preheat the oven to 350 degrees with the rack in the center. Prepare a 9 x 9 or 8 x 8 square pan with non stick cooking spray.
Make the batter. Beat the butter and sugar in an electric stand mixer until combined. Add the eggs and vanilla and beat until fluffy. Combine the dry ingredients in a separate mixing bowl. Add to the butter mixture, ⅓ at a time, alternating with the sourdough starter and sour cream. Mix gently until just combined.
Make the topping. To make the topping, cut the butter into the flour until it is the size of peas. Stir in the brown sugar and cinnamon.
Assemble. Layer half of the cake batter into the prepared pan. Sprinkle half of the topping on the cake batter. Gently spread the remaining cake batter on top, and top with the rest of the topping.
Bake. Bake for 40-45 minutes, until a toothpick inserted into the center comes out clean. Cool in the pan for at least 15 minutes. Serve in squares directly out of the pan.