Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan with cooking spray.
Combine sourdough starter, flour, sugar, oil, baking powder, salt, and cinnamon in a bowl; beat with an electric mixer until smooth and thick. Pour batter into the prepared pan.
Mix brown sugar, softened butter, and cinnamon in a bowl until evenly moistened. Sprinkle topping over the cake batter.
Bake in the preheated oven until a toothpick inserted in the center comes out clean, about 30 minutes.
Meanwhile, mix melted butter, milk, and vanilla in a bowl with a fork until smooth. Drizzle glaze over the coffee cake while the cake is still hot.
