Place an oven rack in the center position and preheat your oven to 325°F (163°C). Place a cupcake liner in each of the 12 cups of your muffin tin.
Add the chocolate sandwich cookies to a food processor and pulse until fine crumbs form. Mix with melted butter until the crumbs hold together when pinched, then divide evenly among the liners and press firmly into the bottom.
Bake the crusts for 5 minutes to help them set and stay crisp under the filling. Remove from the oven and set on a cooling rack, leaving the oven on at 325°F.
Microwave 1/2 cup semisweet chocolate chips on 50% power in 30-second bursts, stirring after each, until smooth. Set aside for about 5 minutes to cool slightly but stay pourable.
Beat the softened cream cheese with a hand mixer on medium speed for about 2 minutes until completely smooth, scraping down the sides and bottom of the bowl.
Add the granulated sugar and beat on medium-low for 1 minute. Sift the cocoa powder over the bowl, add the salt and optional espresso powder, and beat on low until uniform in color.
Mix in the sour cream and vanilla on low until combined. Add the eggs one at a time, mixing on low for just 15 to 20 seconds after each, then scrape down the bowl.
Pour the cooled melted chocolate into the batter and mix on the lowest speed, or fold gently with a spatula, until glossy and evenly colored with no streaks remaining.
Spoon or pour the batter over the crusts, filling each liner about three-quarters full. Tap the tin gently on the counter and bake at 325°F for 18 to 22 minutes, until edges are set and centers have only a slight jiggle.
Turn the oven off, crack the door open, and let the cheesecakes sit inside for 10 minutes. Move the tin to a cooling rack and cool at room temperature for about 1 hour, then cover loosely and refrigerate for at least 4 hours or overnight.
Place the remaining 1/2 cup chocolate chips in a small heatproof bowl. Warm the heavy cream until it just begins to steam, pour over the chocolate, and let sit for 2 minutes. Stir slowly until smooth and shiny, then let thicken for 5 minutes. Spoon about 1 to 2 teaspoons over each chilled cheesecake and spread gently to the edges.
Refrigerate the topped cheesecakes for 15 more minutes so the ganache sets before serving.