Preheat the oven to 350°F (175°C) and grease a 9×13-inch baking dish.

Combine yellow cake mix, eggs, milk, and vanilla in a mixing bowl until just blended.

Mix brown sugar and cinnamon in another bowl. Layer half of the cake batter into the greased dish, sprinkle half of the cinnamon mixture, then pour on the remaining batter and top with the rest of the cinnamon mixture.

Bake for 25-30 minutes. Test doneness with a toothpick; it should come out clean.

Cool the cake slightly, then poke holes into it using a fork. Drizzle with the powdered sugar and melted butter mixture for extra sweetness.