Chocolate Mousse:

1. Start by making a ganache. Add the chocolate and 1 1/2 cups (360g) of heavy cream into a microwave safe bowl, and microwave in 20 second increments, stirring in between, until the chocolate is fully melted and a smooth ganache forms. If there is any unmelted chocolate, microwave the ganache for 10-15 seconds until all the chocolate has melted. Let the ganache rest for 5 minutes while you whip the cream
2. Add the remaining 2 cups (480g) heavy cream to a large bowl. This cream must be straight from the fridge and chilled. Use an electric hand mixer to whisk the cream for a few minutes until it reaches soft / medium peaks. This means the cream will thicken up and form "ribbons" behind the mixer attachments, and when you turn off the mixer and lift it, the cream will form a soft peak which slowly falls back down. Stop whisking immediately at this stage
3. Add about a third of the whipped cream into the ganache (which should not be hot at this point), and use a rubber spatula to very gently and very slowly fold it in. Be extra gentle, and stop when they are mostly combined
4. Add the rest of the whipped cream to the mixture in 1-2 batches, and continue to gently and slowly fold. You want to fully mix the two without knocking out the air in the cream. If you see any lumps of whipped cream, press down on them with your spatula to break them. Stop mixing as soon as the cream has fully mixed with the ganache. You should have a smooth and fluffy chocolate mousse

Cake base:

1. Line a 9x9" pan with 2 sheets of parchment paper extending to the top edges of the pan (you will use this to lift the cake out of the pan once it has cooled down). You can also assemble in a serving dish and serve straight from it, like tiramisu
2. Add the milk and instant coffee (if using) to a wide bowl and mix to combine
3. If using graham crackers, dip each cracker into the milk quickly (in and out) and add a layer at the bottom of the pan. Repeat and add another layer on top. Fill any gaps by trimming the crackers
4. If using bourbon cream biscuits, dip each biscuit into the milk for ~2 seconds per side (they are thicker and need more time to absorb the milk), and add one layer on the bottom of the pan. Fill in any gaps by trimming the biscuits

Assembly and serving

1. Pour the chocolate mousse on top of the chocolate cake base and spread it out. You can lightly tap the pan to get rid of any air bubbles
2. Cover with cling wrap (it shouldn't touch the mousse) and refrigerate for 4-6 hours or until the mousse is fully set
3. Once the cake has chilled, place it in the freezer for 10-15 minutes to firm it up. Then use the parchment paper to lift the cake out of the pan. Carefully flip it upside down on your hand (you can place a paper towel on your hand to avoid sticking) and remove the parchment paper. Then flip it back and place on a serving stand
4. At this point you can add the optional ganache on top. Add the chocolate and cream for the ganache to a bowl and microwave in 20 second increments, stirring in between, until a smooth ganache forms. Let it rest for 3-5 minutes so it's still runny but not hot. Pour the ganache on top of the chilled cake and quickly spread it out
5. Slice, serve and enjoy!

Video Tutorial:

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