Preheat the oven to 350 degrees. Grease two 12 well mini muffin tins or spray with non-stick spray. Set aside.
In the bowl of a stand mixer, beat butter and sugar until well-combined, about 1-2 minutes. Add egg and vanilla and beat until fully combined, stopping to scrape down the sides of the bowl if necessary.
Stop the mixer and add the flour, cocoa powder, salt and baking powder. Mix until everything is fully combined and the dough is uniform in color.
Using a small cookie scoop, scoop out the dough into each muffin tin well, pressing it up the sides and creating an indentation in the center.
Bake for about 10-11 minutes or until the dough is just set.
Meanwhile, make the ganache: Heat the heavy cream in a microwave-proof bowl for about 45 seconds. Add the chocolate chips to the hot cream and let sit for about a minute. {If you're adding extracts or liqueur, you may do so at this point.} Whisk until smooth and fully melted. Spoon the ganache mixture into a zip top bag -- you'll use this to pipe the filling into each cookie cup.
Remove the cookie cups from oven. If the centers have puffed up, use the back of a tablespoon to press down the center again while it's still warm. Let cool for about 10-15 minutes.
When you're ready to pipe in the filling, snip one corner of the zip top bag and squeeze the ganache into each cup, dividing evenly among all 24.
Top each cup with a strawberry slice or desired topping and garnish with mint leaf if desired.