In a mixing bowl, combine plain flour, unsalted butter, egg yolk, and brown sugar until the mixture resembles coarse crumbs. Transfer into a tart pan and press evenly across the bottom and up the sides. Preheat oven to 350°F (175°C) and blind bake the crust for about 15-20 minutes until lightly golden.

In a saucepan, heat full-fat milk with the split vanilla bean over medium heat until it begins to boil. In a separate bowl, whisk egg yolks, brown sugar, and cornstarch until smooth. Gradually pour hot milk mixture into egg mixture, whisking constantly to avoid curdling.

Return the mixture to the saucepan over low heat and continue stirring for about 5-7 minutes until the custard thickens to a pudding-like consistency, ensuring it doesn’t boil. Remove from heat and pour custard into pre-baked crust, smoothing the top with a spatula.

Allow the tart to cool at room temperature for about 30 minutes, then cover with plastic wrap and refrigerate for at least 2 hours, or until fully set.

Once set, wash and thinly slice the red and black plums. Arrange slices over the top of the custard, creating a visually appealing pattern.

Garnish the tart with fresh mint leaves if desired. Serve chilled, either sliced into wedges or removed from the tart pan for a stunning presentation.