For the brownies: Preheat the oven to 325 degrees F. Line a mini-muffin pan with paper liners.
Melt the butter, oil and chocolate discs together over a double boiler, stirring until melted and smooth. Remove from the heat.
In a medium bowl whisk together the flour with the cocoa powder, salt and baking soda.
Stir the brown and granulated sugars and vanilla into the chocolate mixture with a wooden spoon. Add the eggs and beat vigorously until fully incorporated and the batter is thick and glossy. Fold in the dry ingredients until just incorporated.
Spoon the brownie batter into the cupcake liners, coming about three-quarters of the way up the sides. Set aside.
For the cheesecake topping: Using an electric mixer, beat together the cream cheese with the granulated sugar, sour cream, vanilla and egg until well incorporated. Put half of the cheesecake topping into a pastry bag or resealable plastic bag. Use a bit of pink food coloring to dye the remaining topping; place into a second pastry bag.
Snip the tips of the pastry bags and pipe 3 to 4 medium dots of each color on top of the brownie batter. Use a toothpick to gently swirl the cheesecake batter and cover the brownie batter completely. Bake for about 15 minutes, then cool completely.
For the buttercream frosting: Sift the confectioners' sugar into a medium bowl. In a 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; 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.)
Remove the brownies from the pan and decorate with the buttercream frosting and candy or cookies as desired.