Make the crust: Preheat oven to 350°F (175°C). Crush Speculoos cookies finely using a food processor or by placing in a zip-lock bag and crushing with a rolling pin. Mix crushed cookies with melted butter until evenly combined. Press mixture firmly into a 9-inch pie dish to form a crust. Bake for 10 minutes, then set aside to cool.

Prepare the filling: In a medium saucepan, whisk together egg yolks, granulated sugar, heavy cream, dark brown sugar, cornstarch, and salt until smooth. Gradually whisk in whole milk.

Cook over medium heat, stirring constantly, until mixture thickens and comes to a boil (about 8–10 minutes). Continue cooking and stirring for 1 more minute.

Remove from heat and stir in butter until melted and smooth.

Pour filling into the prepared cookie crust. Cover surface with plastic wrap to prevent a skin from forming. Chill in the refrigerator for at least 4 hours or until fully set.

Make the topping: Beat cold heavy cream with powdered sugar and vanilla until stiff peaks form. Spread whipped cream over chilled pie.

Garnish with crushed Speculoos cookies if desired.