Adjust oven rack to lower-middle position and preheat to 375 degrees.
Toast walnuts over medium heat in a small skillet, shaking or stirring occasionally, until golden brown, about 5 minutes. Reserving the 16 walnut halves, transfer remaining nuts into a food processor, process until minced. Add cookie and sugar, process until mixture is finely ground. Drizzle in butter, pulse until incorporated. Transfer mixture to 11” tart pan with removable bottom, tamp crumbs into pan and up sides. Bake until fragrant and golden brown, about 10 minutes. Remove from oven and cool.
Place cream, chocolate chips and coffee in a 1 quart microwave safe bowl, microwave on high power until cream is hot and chocolate chips have mostly melted, about 3 minutes. Whisk until chocolate mixture is smooth. Pour into shell and refrigerate until firm. Can be done one day ahead. No more than 2 hours before serving, bring to room temperature. Garnish with walnut halves, slice and serve with whip cream.