Special equipment: a 24-cup mini-muffin tin; a pastry bag or resealable plastic bag
Candies or cookies for decorating, such as pirouette cookies, white, pink or chocolate curls, dragees, edible glitter, chocolate cigars or wafer cookies
For the brownies: Preheat the oven to 325 degrees F. Spray a 24-cup mini-muffin pan very lightly with nonstick cooking spray and line the bottoms with small rounds of parchment.
In a double boiler, melt the butter and the chocolate together, stirring often to avoid scorching. Remove from the heat once everything is melted and combined.
In a big bowl, whisk together the granulated sugar, brown sugar, eggs and egg yolk. Whisk in the salt and vanilla.
Slowly add the chocolate mixture to the egg mixture to temper it and whisk just to combine.
Gradually stir in the flour and mix until everything is combined and smooth. Stir in the walnuts if using. Set aside.
For the cheesecake topping: With an electric mixer, beat the cream cheese with the granulated sugar and salt until smooth, then add the sour cream, vanilla and egg, and beat together just until incorporated. Transfer the cheesecake topping to a pastry bag or resealable plastic bag.
Use a small ice cream scoop to add the brownie batter to the muffin pan (it should come halfway up the sides). Snip the tip off the pastry bag and pipe some cheesecake topping on top. Bake until a toothpick inserted in the center comes out clean, about 15 minutes. Let cool completely in the pan.
For the buttercream frosting: Sift the confectioners' sugar into a medium bowl. In a stand mixer fitted with the paddle attachment, beat the butter on medium-high until smooth and light, about 1 minute. Add all but 1/4 cup of the confectioners' sugar and mix on low just to combine. Add the vanilla and increase the speed to high, then beat until light and fluffy, 1 to 2 minutes. (Add more confectioners' sugar or a bit of milk depending on how stiff or loose you want the frosting.)
Frost the brownie bites with buttercream frosting, and decorate as desired with candy or cookies.