Beat the butter, cream cheese, brown sugar, granulated sugar and salt in a medium bowl with a mixer on medium speed until smooth and creamy, about 2 minutes. Beat in the egg and vanilla until combined. Reduce the mixer speed to low and beat in the flour until incorporated. Beat in 2 ounces of the chopped chocolate. Cover with plastic wrap and refrigerate until the dough is no longer sticky, about 1 hour.
Coat a 24-cup mini-muffin pan with cooking spray. Divide the dough between the cups (about 1 heaping tablespoon each); press into the bottoms and up the sides to create cups. (If the dough becomes sticky, dampen your fingers.) Refrigerate until firm, 30 minutes.
Preheat the oven to 350 degrees F. Bake the cookies until browned around the edges and dry in the centers, 15 to 20 minutes. Let cool 5 minutes in the pan, then use the back of a small spoon to gently indent the center of each. Use an offset spatula to remove the cookies from the pan; transfer to a rack to cool completely.
Meanwhile, put the remaining 6 ounces chopped chocolate and 3 tablespoons heavy cream in a microwave-safe bowl; microwave in 30-second intervals, stirring, until smooth. Let stand until thick like hot fudge, 10 minutes. (If the chocolate is too thick, gradually stir in 1 more tablespoon heavy cream. If the chocolate hardens, return to the microwave.) Spoon the chocolate into the cookies and top with the candies. Let cool completely.