1. Whisk together sugar, cornstarch, cocoa, and 1/8 teaspoon salt in a medium saucepan. Combine cream, milk, and vanilla in a liquid measuring cup.

2. Pour 1 cup cream mixture into sugar mixture, and whisk until cornstarch is dissolved. Add remaining cream mixture, and whisk until smooth. Bring to a boil, whisking constantly, and cook until thick, about 5 minutes. Reduce heat to low, and add chocolate. Whisk until chocolate melts, about 1 minute.

3. Remove from heat. Add mint extract and butter, and whisk until butter melts. Quickly divide pudding, whisking as you work to prevent sides from setting up, among eight 4-ounce glasses or glass flowerpot votive candle holders, leaving about 1/2 inch of space at the top of each. Refrigerate until set, about 1 1/2 hours.

4. Just before serving, pulse cookies in a food processor until finely ground. Top each pudding with 1 tablespoon ground-cookie "dirt," and garnish with a planted mint sprig. Serve chilled.