In a small bowl, sprinkle the gelatin powder evenly over 1/4 cup cold water. Let it sit undisturbed for 5-10 minutes to bloom and soften. It will become thick and jelly-like.

In a medium mixing bowl, whisk the egg yolks and 1/2 cup granulated sugar together until the mixture is pale yellow and creamy. Set aside.

In a medium saucepan, combine the whole milk and the scraped vanilla bean (or vanilla extract if using). Heat over medium heat until small bubbles appear around the edges, but do not let it boil vigorously. Remove the vanilla bean pod if using.

Slowly temper the hot milk into the egg yolk mixture: gradually pour about 1/3 of the hot milk into the egg yolks, whisking constantly to prevent the eggs from scrambling. This warms the yolks gently. Then, pour the tempered egg mixture back into the saucepan with the remaining hot milk.

Continue to cook over medium-low heat, stirring constantly with a rubber spatula or wooden spoon, until the mixture thickens enough to coat the back of a spoon (it should reach about 175-180°F or 80-82°C on an instant-read thermometer). Do not allow it to boil, or the eggs will curdle. Immediately remove from heat.

Pour the hot custard through a fine-mesh sieve into a clean large mixing bowl to ensure a smooth texture and remove any potential cooked egg bits or the vanilla bean pod. Add the bloomed gelatin to the hot strained custard and whisk vigorously until the gelatin is completely dissolved.

Set the bowl of custard over an ice bath (a larger bowl filled with ice and a little water). Stir occasionally to cool the custard quickly and evenly. Continue stirring until the custard cools down to room temperature and just begins to thicken slightly, about 15-20 minutes. It should be cool to the touch but not yet set.

In a separate, cold medium mixing bowl, using an electric mixer (stand or hand-held) with a whisk attachment, whip the very cold heavy cream on medium-high speed until soft peaks form. The cream should hold its shape but still be soft and billowy. Be careful not to over-whip it into stiff peaks, which can make the Bavarian cream grainy.

Once the custard is cooled and slightly thickened, gently fold about one-third of the whipped cream into the custard to lighten it. Then, add the remaining whipped cream and fold gently until just combined and no streaks of cream remain. Be careful not to deflate the cream's airiness.

Divide the Bavarian cream among individual serving dishes (ramekins, wine glasses, or dessert bowls), or pour it into a lightly oiled 6-cup mold. Cover loosely with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, until fully set and firm.

About 30 minutes before serving, gently wash and prepare the mixed berries. If using strawberries, hull and slice any large ones. In a bowl, combine the berries with the 2-3 tablespoons of granulated sugar (adjust to your sweetness preference) and the fresh lemon juice. Stir gently and let them macerate at room temperature to allow their juices to release and create a light syrup.

To serve, if using a mold, quickly dip the bottom of the mold in warm water for a few seconds (being careful not to get water into the cream), then invert onto a serving plate. If using individual dishes, simply scoop or serve directly from the dish. Top generously with the macerated berries and their delicious juices. Garnish with fresh mint leaves and serve immediately.