1. Preheat oven to 375°F; line a baking sheet with foil. Make crust: Combine wafers, cocoa powder and sugar in a food processor. Process until finely ground, about 20 seconds. Add butter and pulse until mixture just clumps together. Transfer to a 9-inch tart pan with a removable bottom and press firmly onto bottom and sides to form a crust. Place on lined baking sheet and bake until slightly firm to the touch and fragrant, 7 to 10 minutes. Remove baking sheet to a wire rack; reduce oven temperature to 350°F.
2. Make filling: Combine all ingredients in a medium bowl; whisk until blended. Place baking sheet with crust on oven rack and ladle just enough filling into crust to come up to the rim. (Do not overfill.) Bake until a knife inserted into center of tart comes out clean, about 45 minutes. Cool slightly, then chill for at least 2 hours before serving.