Prepare the Brownie Base: Preheat your oven to 180°C (160°C fan) and grease and line a 20cm (8-inch) square baking tin with baking parchment, leaving some overhang to help you lift it out later.

Melt Chocolate and Butter: In a heatproof bowl set over a saucepan of gently simmering water (a bain-marie), melt the 150g of dark chocolate and the butter together, stirring until smooth. Remove from the heat and set aside to cool slightly for about 5 minutes.

Make the Batter: In a large bowl, whisk the caster sugar and eggs together for 2-3 minutes until pale and slightly fluffy. Stir in the vanilla extract and the cooled chocolate-butter mixture until just combined. Sift the plain flour, cocoa powder, and salt over the wet ingredients and gently fold everything together until you can't see any streaks of flour. Be careful not to overmix.

Bake the Brownie: Pour the batter into your prepared tin and smooth the top. Bake for 22-25 minutes. The edges should be set and the centre should have a slight wobble when you gently shake the tin. A skewer inserted into the centre should come out with moist crumbs attached. Let the brownie cool completely in the tin – this is crucial!

Make the Mousse: While the brownie cools, prepare the mousse. Melt the 150g of dark chocolate in a bain-marie or in short bursts in the microwave, then set aside to cool to room temperature. In a separate bowl, whip the 300ml of cold double cream until it holds soft peaks. In another clean bowl, whisk the egg whites until foamy, then gradually add the 50g of caster sugar, whisking continuously until you have stiff, glossy peaks.

Combine the Mousse: Gently fold the cooled melted chocolate into the whipped cream until just combined. Then, add a third of the egg white mixture to lighten the chocolate base before gently folding in the remaining egg whites. I find using a large metal spoon to fold helps keep the maximum amount of air in the mousse.

Assemble and Chill: Once the brownie is completely cool, spread the chocolate mousse evenly over the top. Place the tin in the refrigerator and chill for at least 4 hours, or until the mousse is firm to the touch.

Prepare the Ganache: Place the 100g of finely chopped chocolate in a heatproof bowl. Heat the 120ml of double cream in a small saucepan until it just begins to simmer. Pour the hot cream over the chocolate and let it sit for 5 minutes without stirring. Then, stir gently from the centre outwards until you have a smooth, glossy ganache.

Finish and Chill Again: Let the ganache cool for about 10-15 minutes, then pour it over the set mousse layer, gently tilting the tin to cover the surface completely. Return it to the fridge for at least 1 hour, or until the ganache is set. To serve, lift the brownie out of the tin using the parchment paper and slice into squares with a sharp knife.