Preheat the oven to 180°C/160°C fan/gas mark 4. Grease and line a 20x30cm loose-bottomed brownie tray. For the sponge, add all ingredients to a large mixing bowl and mix until just combined. Tip into the prepared tin and bake in the oven for 15 minutes, until springy and cooked through. Transfer to a cooling rack and set aside to cool.
For the mousse, put the gelatin leaves in a small bowl and cover with cold water. Leave to soften for 6-8 minutes. Meanwhile, blitz the raspberries in a blender until smooth. Strain the raspberries through a fine sieve into a small saucepan and discard the seeds. Add 5g of the sugar and 1 tbsp of water to the raspberries and cook gently for a couple of minutes until the sugar dissolves and the mixture bubbles a little. Remove from the heat.
Once the gelatin leaves are soft, lift them from the water and shake off any excess liquid. Add to the raspberry sauce and stir to melt. Leave the sauce to cool, stirring occasionally.
Add the egg whites to the bowl of a stand mixer with the whisk attachment. Whisk until it forms soft peaks, then slowly add the sugar and continue to whisk for a few more minutes, until they reach firm peaks.
Spoon a third of the egg whites into the raspberry sauce ands mix well. Don't worry about losing air from the egg whites at this stage. Add this mixture to the rest of the egg whites, and now gently fold through until combined to make an airy mousse.
Pile the mousse on top of the cooled sponge, and use a spatula or palette knife to spread out evenly. Chill in the fridge until set for about 3 hours.
Gently heat the remaining raspberries until they release some juice, leave to cool slightly and drizzle over the top. Slice carefully with a hot knife when ready to serve.