Make the chocolate mousse. In a large bowl, mix the room temperature cream cheese, sugar, and milk until smooth and creamy. Add the cocoa powder and mix again until fully combined. Gently fold in the cool whip with a spatula. Be careful not to overmix, or the mousse will deflate and lose its airy texture.

Prepare the chocolate ganache. Place the chocolate chips in a medium heatproof bowl. Microwave the heavy cream until it just begins to boil, about 1 to 2 minutes depending on your microwave. Pour the hot cream over the chocolate chips and cover the bowl with a plate. Let it sit for 5 to 7 minutes. Then whisk slowly from the center outward until the ganache becomes smooth and glossy.

Make the chocolate whipped cream. In a chilled bowl, whip the heavy whipping cream, cocoa powder, and powdered sugar together. Use a hand mixer on medium-high speed until stiff peaks form. This takes about 2 to 3 minutes. Keep it cold until you are ready to assemble.

Assemble the first layer. In a 9x13 inch pan, arrange a single layer of chocolate graham crackers. Break them as needed to fit evenly across the bottom. Spread half of the chocolate mousse over the crackers with an offset spatula.

Add more layers. Drizzle or spread about one-third of the chocolate ganache over the mousse. Add a second layer of graham crackers, then the remaining mousse, and another portion of ganache. Finish with a final layer of graham crackers.

Top and chill. Spread the chocolate whipped cream over the top layer of graham crackers. Cover the pan loosely with plastic wrap and refrigerate for at least 4 hours, or overnight if possible. This resting time is essential because the graham crackers need to soften into cake-like layers.

Garnish and serve. Just before serving, drizzle chocolate sauce over the top and sprinkle with mini chocolate chips. For the cleanest slices, wipe your knife between cuts.