Make the crusts: Pulse the flour, walnuts, granulated sugar and salt in a food processor until finely ground. Add the butter and pulse until the mixture looks like coarse meal. Drizzle in 2 to 3 tablespoons ice water and pulse until the dough starts to come together. Turn out onto a sheet of plastic wrap; shape into a disk, then wrap and refrigerate until firm, about 1 hour.
Preheat the oven to 375 degrees F. Spray an 18-cup mini-muffin pan with cooking spray. Press 1 tablespoon dough into the bottom and up the side of each cup. Refrigerate 20 minutes.
Bake the crusts until golden brown, 20 to 25 minutes, then press the centers of the crusts down with a small spoon. Let cool in the pan 10 minutes, then remove to a rack to cool completely.
Make the filling: Put the chocolate chips in a bowl. Combine 3 tablespoons cream and 1 tablespoon bourbon in a small microwave-safe bowl and microwave until simmering, about 30 seconds; pour over the chocolate chips and let stand 1 minute, then stir until smooth. Let cool slightly.
Spoon the filling into the crusts and let set, about 20 minutes. Whisk the remaining 1/4 cup cream and 1 tablespoon bourbon with the confectioners' sugar until soft peaks form; spoon onto the tartlets and top with the candied walnuts.