For the pumpkin mousse squeeze juice from 4 oranges. Caramelize sugar and deglaze with juice from oranges and lemon.
Add pumpkin and cinnamon and cook over low heat until soft, about 15 minutes. Finely puree and let cool. Melt white chocolate and whipping cream over a water bath or in the microwave.
Add the cooled pumpkin puree. Strain the mixture through a fine hair sieve and pour into a 0.5 L iSi device.
Screw on an iSi cream capsule and shake vigorously. Chill in the refrigerator for at least 6 hours.
For the brownie, melt the chocolate and butter. Cream together the granulated sugar and eggs. Add the chocolate-butter mixture and flour.
Pour the mixture onto a greased baking tray and sprinkle with whole hazelnuts. Bake for about 20 minutes at 180 °C until it comes away easily from the edges.
If necessary, leave in the oven for 10 minutes. Arrange the brownie with the pumpkin mousse and serve immediately.