Infuse the milk: Heat the milk in a saucepan. If using a vanilla bean, split it, scrape the seeds into the milk, and add the pod. Heat until just about to boil. If using vanilla extract, add it later.

1. Prepare the egg mixture: Whisk together egg yolks and granulated sugar until smooth and pale.
2. Cook the custard: Gradually add the hot milk into the egg mixture while whisking constantly. Pour the mixture back into the saucepan and cook over low heat, stirring constantly, until it thickens slightly and coats the back of a spoon (about 5-7 minutes). Remove from heat.
3. Bloom the gelatin: Sprinkle gelatin over water and let it sit for 5 minutes. Gently heat until liquid, then stir it into the custard mixture until dissolved.
4. Chill the custard: Cool the custard to room temperature, then refrigerate for 20-30 minutes, or until it begins to thicken but is not fully set.
5. Whip the cream: In a separate bowl, whip the heavy cream with powdered sugar until soft peaks form.
6. Combine the cream and custard: Gently fold the whipped cream into the cooled custard until smooth and fully incorporated.
7. Set the Bavarois: Pour into individual molds or a large serving dish. Smooth the top and refrigerate for at least 4 hours or overnight to set.
8. Serve: Unmold the Bavarois carefully or serve directly from the dish. Garnish with fresh berries or grated chocolate if desired.