Over medium heat, bring chestnuts, cream, salt, vanilla and allspice to a simmer in a small saucepan. Cook, stirring constantly until chestnuts are falling apart and milk has thickened, about 10-15 minutes. Be careful to keep milk from scalding.
Transfer mixture to a food processor or blender and puree until smooth. Set aside.
In a medium bowl with an electric mixer, beat yolks on high speed for 2 minutes.
With mixer continuing to run, slowly add granulated sugar and maple syrup until pale and thick, for approximately another 2 minutes.
Lower mixer to medium speed. Slowly pour puree until smooth.
Transfer cream and egg mixture back to small saucepan.
Over medium low heat, whisk continuously for 1- 2 minutes or less. This step is to help “cook” the egg. Be careful as it will scald if you’re not careful. Remove from heat.
Set aside to cool.
In a fresh mixing bowl, whisk cream to soft peaks.
Use a spatula to gently fold in cooled mixture.
Place a fresh dollop of mousse on a cooled pavlova. Sprinkle with chopped nuts and drizzle with chocolate. Serve immediately.
Store any extra mousse in an airtight container for up to 5 days.