Preheat your oven to 180°C / 350°F. Spray an 8-inch springform pan with oil spray and line the bottom with baking paper.
Fill a pot about a quarter of the way with water. Bring to a gentle boil. Place a heatproof bowl on top and add the butter, chocolate, and brown sugar. Stir until all the ingredients have melted. Take off the heat and add the eggs, whisk to combine. Add the flour and whisk until the batter is smooth.
Add to the springform pan and bake for 20 minutes. Once baked, allow to cool completely at room temperature. Carefully run a knife around the edge of the brownie to loosen from the springform pan. Clean the inside of the springform pan. Place a piece of baking paper on bottom and place the sides on top. Clip to tighten.
Spray the side of the pan with oil spray and place a large piece of acetate measuring 65cm (26-inches) in length on the inside. The oil will help it stick to the pan. Place the cooled brownie in the pan and set aside.
Add the milk and dark chocolate in a heatproof bowl over a double boiler. Stir until melted.
Take off the heat and add the egg yolks. Mix using a spatula. The mixture will instantly thicken and become almost dough like. Set aside.
Add gelatine leaves to cold water to soak for 5 minutes. Take out and squeeze out excess water. Place in a jug with 1 cup of the cream which has been heated. Stir until gelatine is melted.
Add the hot cream to the chocolate egg mixture and stir until smooth. Set aside.
Add the egg whites to a separate large bowl. Make sure the bowl is clean. Use an electric hand mixer to whip to soft peaks. Add a third of the sugar and whip for one minute. Repeat until all the sugar is in. Once in, whisk on high speed for 2 minutes until meringue is thick and glossy.
Finally, to a separate bowl whisk the remaining cream and vanilla extract together until you reach stiff peaks.
Add the cream to the chocolate module and gently fold through until well combine. Add the meringue and fold through until well combined.
Pour the mixture into the prepared springform pan. Use a spatula to flatten out the top. Chill in the fridge for four hours or overnight.
To remove, run a knife under hot water and dry. Run it around the top of the mouse layer before loosening it from the springform pan.
Carefully lift up the sides of the pan. Gently unwrap it from the acetate and slice to serve.
This cake can be stored in an airtight container for up to three days before it’s unwrapped. It is best served the day it’s unwrapped.