Line 9 x 13-inch pan with nonstick foil.
Preheat oven to 350º.
In a large mixing bowl or bowl of a stand mixer, combine flour, sugar, cocoa, baking soda, and baking powder.
In a large pyrex measuring cup or medium mixing bowl, whisk together eggs, coffee, buttermilk, vegetable oil, and vanilla.
Add the liquids to the dry ingredients and beat for a couple minutes, scraping down the sides as needed.
Pour cake batter into prepared pan and bake 30-40 minutes or until a toothpick inserted into the center comes out clean.
When cake is almost done baking, in a microwave-safe bowl, microwave the sweetened condensed milk and chocolate chips for 1-2 minutes, stopping and stirring a couple times, until mixture is smooth and chocolate chips are melted.
When cake is done, let cool on a wire rack for about 5 minutes.
Then take a wooden spoon and using the handle end, poke rows of holes across the cake. Pour warm filling mixture over the holes, nudging the filling as needed to fill the spaces.
Cool to room temperature before frosting.
