Preheat the oven to 350°F and set the tart cups on a flat baking sheet.
Prepare the brownie batter according to the box directions. Spoon the batter into each tart cup, filling about two-thirds full.
Bake for 20 to 24 minutes, or until the tops are set and a toothpick comes out mostly clean.
While still warm, press the center of each brownie gently with the back of a spoon to create a small well.
Allow them to cool completely.
Heat the heavy cream in a small saucepan just until it begins to simmer. Pour the hot cream over the milk chocolate chips and let it sit for one minute. Whisk until smooth and glossy to make a ganache.
Spoon or pipe the ganache into each brownie tart shell, filling the center.
Top each tart with a chocolate cat and a light mist of edible glitter for a sparkling finish.