Place the chopped chocolate in a microwave-safe bowl and melt it in 30-second increments, stirring after every 30 seconds until melted and smooth. Set aside to cool to room temperature.
Place the remaining ingredients (sugar through salt) in the bowl of a large stand mixer fitted with a paddle attachment. Mix on low speed until the sugar is thoroughly moistened, then raise the speed to medium and beat for 1-2 minutes, until light and fluffy.
Turn the mixer speed back to low, and add the chocolate. Once the chocolate is mixed in, raise the speed back to medium and beat for one minute more. If the frosting seems too stiff, add a bit more milk until you get a texture you like. Use immediately, or place in an airtight container with plastic wrap pressed on top. The frosting can be made several days in advance and kept in the refrigerator. Allow to come to room temperature before using.
To assemble the brownies, spread the frosting all over the top of the cooked brownies, and create swirls with the back of a spoon. Sprinkle your toppings all over the top: use a variety of shapes and colors for maximum fun! Press down gently on the larger items to embed them in the frosting.
If you have time, chill the brownies before cutting them—having the brownies and frosting firm will give you really clean cuts. Before serving, top each brownie with a piece of swirled chocolate bark. For the best taste and texture, enjoy these brownies at room temperature. They will keep for up to two weeks in a container in the refrigerator.