In a large mixing bowl, melt your butter, then sift in your cocoa powder, mixing until smooth. Add in your sugar and mix it in briefly, before adding the eggs and vanilla, mixing in between each addition. Sift in the flour, salt and baking powder and fold into your chocolate mixture until combined. Pour your brownie mix into your prepared springform tin, levelling the top, and bake for 25-30 minutes until done. Leave to cool completely.

Before you make your mousse, remove your brownie from the springform and level the top. Then flip it upside down, remove the paper on the bottom and return it to the springform base, cut side down. Wrap the brownie in a collar of acetate before putting the springform side back on the tin to hold everything in place. Set aside while you make the mousse.

Now to make the mousse! First, whip up your double cream to stiff peaks. Cover the bowl and chill whilst you make the rest of the mousse.

In a large bowl, tip in your egg yolks, espresso (or water if you don't want to use espresso), salt, butter and 1 tsp of sugar. Place your bowl over a small saucepan of gently simmering water and whisk. First, your butter has to melt, so your mixture may initially look like a curdled mess. Don't worry, just keep heating gently and moving it all around so the yolks don't cook on the bottom. Once your butter has melted, start whisking your mixture constantly until it has doubled in volume. Keep checking the temperature of your mixture regularly, whisking continuously, until it reaches 71C/160F, then remove from the heat. Add in your chopped chocolate and whisk until it has melted and your mixture is totally smooth. Set aside to cool to room temperature, whisking occasionally.

In a separate clean bowl, place your egg whites, cream of tartar and the remaining sugar, whisking it all together. Place over a small saucepan of gently simmering water and heat gently, whisking constantly (really important for the egg whites!) until the temperature of your mixture reaches 71C/160F. Remove from the heat and immediately tip into the bowl of your stand mixer. Using a whisk attachment, beat your egg whites on high until they reach stiff peaks and have cooled right down to room temperature.

Add a third of your egg whites mixture into your cooled chocolate mixture and use to loosen the chocolate mix. Then tip in the remaining egg whites and fold very gently and carefully to avoid knocking out all the air you've incorporated in. Once fully combined, repeat the process with the chilled whipped cream, ensuring everything is completely mixed together and homogenous.

Very gently, pour your mousse mixture over your brownie base, carefully encouraging it evenly all the way around the brownie base. Give the tin a gentle shake to spread the mousse out and smooth the top, then place the springform in the fridge for the mousse to firm up and set completely. This will take at least 3-4 hours, but I prefer leaving it overnight.

In a small bowl, soak your gelatine leaf in cold water for 10 minutes.

In a small saucepan, combine all your glaze ingredients, except for the gelatine, and heat gently until melted and fully combined. Remove your gelatine leaf from the cold water and squeeze out any excess water, then add it to your small saucepan. Continue to heat gently until your gelatine has melted, then leave to cool for a few minutes. Remove the mousse cake from the fridge and carefully pour the cocoa glaze over the top. Return the mousse cake to the fridge for at least another 30 minutes to set.

When you are ready to serve your mousse cake, remove the springform side and carefully peel away the acetate collar. Transfer your mousse cake to a serving plate, garnish with strawberries and serve.