Preheat your oven to 350°F (175°C) and prepare a 9x13 inch cake pan by greasing it well. Mix the chocolate cake according to package instructions, then pour the batter into the pan. Bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.

Once the cake is completely cooled, take a fork and gently poke holes all over the surface, spaced about 1 inch apart to allow the sweetened condensed milk ganache to soak into the cake.

In a small saucepan over medium heat, warm the sweetened condensed milk until it's hot but not boiling, stirring occasionally. Once heated, carefully pour it evenly over the poked cake.

Using a hand mixer, whip 1 cup of heavy cream in a mixing bowl until stiff peaks form—this should take about 3-5 minutes. Gently fold in the mini marshmallows.

Spread the fluffy marshmallow whipped cream evenly over the entire surface of the cake, covering the ganache layer.

Place the finished cake in the refrigerator and chill for at least 4 hours, or ideally overnight, to allow the flavors to meld and the layers to set.