Preheat oven to 350˚F & spray a half sheet pan with non-stick cooking spray.
Place cake mix, pudding mix, eggs, oil, milk, sour cream & pumpkin into a stand mixer. Beat well until combined; about 1½ minutes. Pour into prepared pan & spread evenly with a spatula.
Melt butter in the microwave, about 30-40 seconds. Add the brown sugar & cinnamon, stir until combined. Drizzle over cake batter. Use a knife to create a marbled look by swirling back & forth through the batter.
Bake for 30-35 minutes until cooked through then remove from oven. Combine powdered sugar & milk into a bowl & stir until smooth. Drizzle over warm cake. Keep refrigerated until ready to serve.