Prepare the pan: Heat the oven to 350°F or 180°C. Grease a 9 by 13-inch baking pan.
Mix the batter: Whisk the flour, sugar, cocoa, baking powder, baking soda, and salt. Add the eggs, milk, oil, and vanilla, then stir until combined. Mix in the hot coffee or water.
Bake: Pour the batter into the pan and bake for 30 to 35 minutes, until a toothpick comes out with a few moist crumbs.
Poke the cake: Rest the cake for 10 minutes. Use the rounded handle of a wooden spoon to make deep, evenly spaced holes.
Add the filling: Warm the condensed milk and chocolate chips in short microwave bursts, stirring until smooth. Pour over the warm cake.
Make the topping: Whip the cold cream, powdered sugar, cocoa, and vanilla until medium-stiff peaks form.
Chill and serve: Cool the cake completely, spread on the topping, and refrigerate for at least 2 hours before slicing.