Salted Coffee Custard Pie

Crust

Step 1

Place a rack in middle of oven and preheat to 350°. Butter pie dish with unsalted butter. Pulse 2 cups (180 g) old-fashioned oats, ¼ cup (31 g) all-purpose flour, ¼ cup (packed; 50 g) dark brown sugar, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a food processor until oats are coarsely ground. Drizzle in 6 Tbsp. unsalted butter, melted, slightly cooled; pulse just until mixture comes together. Transfer to prepared dish; wipe out and reserve food processor. Press crust firmly and evenly across bottom and up sides of dish with a flat-bottomed measuring cup. Bake until golden and surface looks dry, 25–30 minutes. Transfer crust to a wire rack; reduce oven temperature to 325°.

Custard and assembly

Step 2

Pulse 1 cup (packed; 200 g) dark brown sugar, 2 Tbsp. all-purpose flour, 2 Tbsp. instant espresso powder, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt in food processor until no lumps remain, about 1 minute. Add 3 large eggs, ½ cup (1 stick) unsalted butter, melted, slightly cooled, and 1 Tbsp. vanilla bean paste or vanilla extract and pulse until smooth. Add 1 cup cold-brew coffee concentrate and 2 cups heavy cream and pulse until well combined.

Step 3

Pour custard into crust (it’s okay if crust is still warm). Bake pie until edges of filling are set and slightly puffed but center still wobbles a little when gently shaken, 40–50 minutes (filling will continue to set after baking). Transfer to a wire rack; let cool at least 3 hours, or, preferably, chill 12 hours.

Step 4

Using an electric mixer, beat ½ cup mascarpone, 4 Tbsp. granulated sugar, remaining 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, ⅔ cup heavy cream, and 1 tsp. vanilla bean paste or vanilla extract in a medium bowl until soft peaks form. Spoon whipped cream mixture over pie; swirl lightly to create divots and peaks.

Step 5

Just before serving, dust with instant espresso powder if desired; sprinkle generously with flaky sea salt.

Do ahead: Pie (without espresso powder and sea salt garnishes) can be made 1 day ahead. Cover loosely and keep chilled.