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1. Preheat oven to 350°F (175°C). Wrap the bottom and sides of a 9-inch springform pan tightly with heavy-duty aluminum foil to prevent water from seeping in during the water bath.
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2. Prepare the Crust: In a medium bowl, combine chocolate cookie crumbs, ¼ cup granulated sugar, and melted butter. Press the mixture firmly into the bottom of the prepared springform pan. Bake for 8-10 minutes, then remove from oven and let cool slightly.
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3. Prepare the Cheesecake Filling: In a large bowl, using an electric mixer on medium speed, beat the softened cream cheese until smooth. Gradually add 1 ½ cups granulated sugar and beat until well combined, scraping down the sides of the bowl.
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4. Add the ½ cup cocoa powder and the melted, cooled dark chocolate to the cream cheese mixture. Beat until fully incorporated and smooth.
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5. Add the eggs one at a time, beating on low speed after each addition just until combined. Do not overmix. Stir in the sour cream, vanilla extract, and pinch of salt until just blended.
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6. Pour the cheesecake filling over the cooled crust in the springform pan.
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7. Prepare Water Bath: Place the foil-wrapped springform pan inside a larger roasting pan. Carefully pour hot water into the roasting pan until it reaches about halfway up the sides of the springform pan.
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8. Bake the Cheesecake: Carefully transfer the roasting pan to the preheated oven. Bake for 60-75 minutes, or until the edges are set but the center still jiggles slightly when gently shaken. (An instant-read thermometer inserted near the center should read 150-155°F / 65-68°C).
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9. Cooling and Chilling: Turn off the oven, prop the oven door open slightly, and let the cheesecake cool in the water bath for 1 hour. Remove from the water bath, remove the foil, and let cool completely on a wire rack at room temperature. Once cool, cover and refrigerate for at least 6 hours, preferably overnight, until thoroughly chilled.
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10. Prepare the Chocolate Mousse Topping: In a cold bowl with cold beaters, whip the heavy cream on medium-high speed until soft peaks form. Gradually add the powdered sugar, cocoa powder, and vanilla extract, whipping until medium-firm peaks form. Gently fold in the melted and cooled 2 ounces of dark chocolate.
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11. Assemble and Serve: Once the cheesecake is thoroughly chilled, gently run a thin knife around the edge of the pan before releasing the springform sides. Spread the chocolate mousse evenly over the top of the cheesecake. Garnish with chocolate shavings or curls, if desired.
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12. Return the cheesecake to the refrigerator for at least 30 minutes for the mousse to set slightly before serving. Store leftovers covered in the refrigerator.
