Preheat your oven to 350°F (175°C). In a large mixing bowl, combine the chocolate cake mix with ingredients as per the package instructions. Blend until smooth and pour the batter into a greased 9x13-inch baking dish. Bake for 25-30 minutes, or until a toothpick comes out clean.

Once the cake has cooled for about 10-15 minutes, use a fork to poke holes all over the surface, about 1 inch apart.

In a small saucepan, heat sweetened condensed milk over medium heat until it begins to steam. Pour the warm milk evenly over the poked cake. Let the cake sit for at least 30 minutes.

In a large mixing bowl, use an electric mixer to beat heavy whipping cream until soft peaks form, about 2-3 minutes. Fold in mini marshmallows.

Spread the marshmallow whipped cream mixture over the top of the cake, smoothing it out gently with a spatula.

Refrigerate the cake for at least one hour before serving. Slice into squares and plate each piece.