In a mixing bowl, combine powdered sugar, pastry flour, almond flour, and salt. Cut in very cold unsalted butter until the mixture resembles coarse crumbs.

Add the egg to the flour mixture and mix until just combined. Press the dough into a greased tart pan and chill in the refrigerator for 30 minutes.

Preheat your oven to 180°C (350°F). Remove the dough and cover it with parchment paper, adding pie weights. Bake for 15-20 minutes until golden brown.

In a saucepan, heat whole milk and granulated sugar over medium heat until steaming. In a bowl, whisk together egg yolks, corn starch, and all-purpose flour until smooth.

Gradually pour the hot milk into the egg mixture, whisking continuously. Return to the saucepan and cook over medium heat until thickened, about 5 minutes. Stir in vanilla bean paste and unsalted butter. Let cool.

Pour the cooled custard into the baked tart shell. Whip heavy cream and mascarpone until soft peaks form and spread over the custard. Garnish with sliced strawberries and chopped pistachios.