Begin by preparing the ganache. In a microwave-safe bowl, combine the chopped chocolate and 1 ½ cups (360 g) of heavy cream. Microwave in 20-second intervals, stirring in between, until the chocolate is completely melted and a smooth ganache is formed. If there are any unmelted pieces, microwave for an additional 10-15 seconds. Allow the ganache to rest for 5 minutes while you whip the cream.

In a large bowl, add the remaining 2 cups (480 g) of chilled heavy cream. Using an electric hand mixer, whisk the cream for a few minutes until it reaches soft to medium peaks. The cream should thicken and form ribbons behind the mixer attachments, and when you lift the mixer, the cream should form a soft peak that gently falls back down. Stop whisking at this point.

Gently fold about a third of the whipped cream into the ganache, ensuring the ganache is no longer hot. Use a rubber spatula and be careful not to deflate the whipped cream. Once mostly combined, add the remaining whipped cream in 1-2 batches, continuing to fold gently until fully incorporated. If you notice any lumps of whipped cream, press them down with the spatula. The final mixture should be smooth and fluffy.

Line a 9x9 inch pan with two sheets of parchment paper, allowing the paper to extend over the edges for easy removal later. Alternatively, you can assemble the cake in a serving dish.

In a wide bowl, mix the milk and instant coffee (if using) together. If using graham crackers, quickly dip each cracker into the milk and layer them at the bottom of the pan. Repeat to create another layer, trimming any excess crackers to fill gaps. If using bourbon cream biscuits, dip each biscuit for about 2 seconds on each side and layer them in the pan, trimming as needed.

Pour the chocolate mousse over the layered base and spread it evenly. Lightly tap the pan to eliminate any air bubbles. Cover the pan with cling wrap, ensuring it doesn't touch the mousse, and refrigerate for 4-6 hours or until the mousse is fully set.

Once the cake has chilled, place it in the freezer for 10-15 minutes to firm up. Use the parchment paper to lift the cake out of the pan. Carefully flip it upside down onto your hand (you can use a paper towel to prevent sticking) and remove the parchment paper. Flip it back over onto a serving stand.

If desired, prepare the optional ganache by combining the chocolate and cream in a bowl and microwaving in 20-second increments until smooth. Let it rest for 3-5 minutes until it's still pourable but not hot. Pour the ganache over the chilled cake and quickly spread it out. Slice, serve, and enjoy!