1. In a medium saucepan, combine the double cream, granulated sugar, and orange zest. Heat over medium and stir until the sugar dissolves completely. Bring to a gentle boil and simmer for 3 minutes.

2. Remove the pan from heat and stir in the strained orange juice and vanilla extract if using. Mix gently to avoid curdling.

3. Strain the mixture through a fine sieve into a jug or bowl. Pour the smooth mixture into ramekins or serving glasses.

4. Let the possets cool slightly at room temperature, then refrigerate for at least 4 hours or until fully set.

5. To make the ganache, add chopped chocolate to a heatproof bowl. Heat the cream until just boiling, then pour over the chocolate. Let sit for 1 minute, then stir into a smooth ganache.

6. Spoon or pour the ganache over each set posset. Chill for an additional 30 minutes to firm the topping.

7. Garnish with orange zest or chocolate shavings just before serving.