Line a 9-inch tart pan with the sweet pastry crust and chill it until firm.

Preheat the oven to 180°C / 356°F and place the lined tart pan on a baking tray.

Whisk the egg, egg yolk, brown sugar, and cornstarch in a mixing bowl until smooth.

Bring the whole milk to a boil in a saucepan.

Gradually whisk the hot milk into the egg mixture to temper it.

Return the mixture to the saucepan and cook over medium heat until very thick, whisking continuously.

Remove the pan from the heat and stir in the salted butter and fleur de sel.

Stir the heavy cream into the warm custard until fully incorporated.

Divide the custard into two equal portions and flavor one portion with vanilla.

Stir most of the pistachio praline into the second custard portion, reserving some for the center and topping.

Pipe the vanilla custard into the pastry crust and pipe the pistachio custard into its center for a marbled effect.

Pipe the reserved pistachio praline into the center of the pistachio custard.

Bake the flan at 180°C / 356°F for 45 to 50 minutes, until the filling is set with a slight central wobble.

Scatter the chopped pistachios over the flan and return it to the oven briefly to toast them.

Cool the flan completely, then refrigerate until chilled and finish with the reserved pistachio praline.