On a lightly floured surface, roll Pâte Sablée into a 13-inch circle (about 1⁄4 inch thick). Carefully transfer to a 10-inch round fluted removable-bottom tart pan, gently pressing into bottom and up sides. Trim excess dough. Dock bottom of dough with a fork. Freeze until firm, about 30 minutes.

Preheat oven to 375°F (190°C).

Top dough with a piece of parchment paper, letting ends extend over edge of pan. Add pie weights.

Bake until edges are golden brown, about 15 minutes. Carefully remove parchment and weights, and bake until bottom is golden brown and dry, 5 to 7 minutes more. Let cool completely.

In a large saucepan, whisk together milk and 1⁄4 cup (50 grams) sugar; cook over medium heat, stirring occasionally, until steaming. (Do not boil.)

In a large bowl, whisk together egg yolks, cornstarch, salt, and remaining 1⁄4 cup (50 grams) sugar. Gradually add warm milk mixture, whisking constantly. Pour egg yolk mixture into saucepan, and cook over medium heat, whisking constantly, until thickened and mixture starts to boil; cook, whisking constantly, for 3 minutes. Strain mixture through a fine-mesh sieve into another large bowl. Stir in butter and vanilla bean paste until butter is melted and mixture is well combined. Cover with plastic wrap, pressing wrap directly onto surface of pastry cream to prevent a skin from forming. Refrigerate until cooled, 30 to 45 minutes.

Whisk cooled pastry cream to loosen. Whisk mascarpone; fold mascarpone into pastry cream until smooth. Spread into cooled prepared crust. Cover with plastic wrap, pressing wrap directly onto surface of filling, and refrigerate for at least 4 hours or up to overnight.

Just before serving, gently remove tart from pan, and top with berries as desired.

In a small microwave-safe bowl, heat preserves and 2 teaspoons (10 grams) water on high until mixture is warm and fluid, about 30 seconds. Using a pastry brush, gently brush preserves mixture onto berries. Garnish with mint, if desired. Refrigerate in an airtight container for up to 3 days.