Make the cupcakes: Preheat the oven to 350˚. Line a 12-cup muffin pan with paper liners. Combine the butter, unsweetened chocolate, cocoa powder and water in a medium saucepan over medium heat. Cook, stirring, until melted and smooth. Remove from the heat and whisk in the sugar, eggs and vanilla until well combined. Whisk in the flour and salt until smooth. Divide the batter evenly among the prepared muffin cups, filling them about three-quarters full.
Bake until the tops of the brownies spring back when lightly pressed and a toothpick inserted into the centers comes out with moist crumbs, 20 to 25 minutes. Let cool 5 minutes in the pan, then remove the brownies to a rack to cool completely.
Meanwhile, make the topping: Combine the semisweet chocolate and heavy cream in a small microwave-safe bowl. Microwave in 30-second intervals, stirring, until smooth and melted. Let stand until thickened slightly, 10 to 30 minutes (it should look like pudding). Spoon over the brownies and gently spread to the edges. Top with the cookie crumbs, gently pressing them into the chocolate.
Tint the vanilla frosting green with food coloring; transfer to a piping bag fitted with a small round tip and pipe onto the brownies to make sprouting grass.