Preheat and prep: Set your oven to 350°F (175°C).
Grease a 9x13-inch baking pan or line it with parchment for easy lifting.
Mix the cake batter: In a large bowl, whisk the cake mix, pumpkin purée, eggs, oil, milk, vanilla, pumpkin pie spice, and brown sugar (if using). Stir just until smooth. Avoid overmixing.
Bake: Pour the batter into the pan and smooth the top.
Bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Cool briefly and poke: Let the cake cool 10–15 minutes. Using the handle of a wooden spoon or a thick straw, poke holes about 1 inch apart all over the cake, going about 3/4 of the way down.
Make the filling: Whisk the sweetened condensed milk, heavy cream, cinnamon, and a pinch of salt until smooth.
Soak the cake: Slowly pour the filling over the warm cake, focusing on the holes. Use a spatula to gently guide the mixture into the holes so it absorbs evenly.
Chill: Let the cake cool to room temperature, then cover and chill for at least 2 hours, preferably overnight.
This sets the texture and deepens the flavor.
Make the topping: Beat the cream cheese, powdered sugar, and vanilla until smooth. In a separate bowl, whip the heavy cream to soft peaks. Fold the whipped cream into the cream cheese mixture.
Add maple syrup if you like a hint of maple.
Frost and finish: Spread the topping over the chilled cake. Dust lightly with cinnamon or pumpkin pie spice. Sprinkle with pecans or pepitas if you want a little crunch.
Slice and serve: Cut into squares and enjoy.
The cake is even better the next day.