Line an 8x8-inch baking pan (about 2 inches high) with 2 sheets of parchment paper, positioning them perpendicular to each other, so that the parchment hangs about 2 inches over all 4 sides of the pan to form a sling.

Place 6 ounces finely chopped semisweet chocolate in a medium bowl. Bring ¾ cup heavy cream to a simmer in a small saucepan over medium-low heat. Pour evenly over the chocolate and let sit for 5 minutes. Whisk until smooth. Refrigerate until ready to use. Meanwhile, prepare the crust.

Break 12 whole graham cracker sheets into rough pieces and place in a food processor. Add 2 tablespoons granulated sugar and ¼ teaspoon kosher salt. Process until the crackers are the texture of sand, about 15 seconds. Add 1 stick melted and cooled unsalted butter and pulse to combine, 6 to 8 (1-second) pulses. (If using graham cracker crumbs, just stir the crumbs and butter together in a medium bowl.)

Transfer the mixture to the baking pan. Use the bottom of a measuring cup or your fingers to press the crumbs into an even layer, making sure to reach the edges and fill the corners. Refrigerate while you prepare the filling.

Beat 1 cup cold heavy cream and ¼ cup of the powdered sugar together in a stand mixer with the whisk attachment on high speed until stiff peaks form, 1 to 2 minutes. Transfer to another bowl.

Beat 8 ounces room temperature cream cheese, the remaining ¼ cup powdered sugar, and ¼ teaspoon kosher salt together in the stand mixer (no need to clean) with the whisk attachment on medium speed until light and very fluffy, scraping down the sides of the bowl with a flexible spatula halfway through, about 3 minutes total.

Transfer 1 cup of the whipped cream into the cream cheese mixture and gently fold together with a flexible spatula until just combined. Dollop onto the crust and spread into an even layer.

Drizzle the ganache over the cream cheese mixture and spread into an even layer. Refrigerate for 5 minutes.

Drop small dollops of the remaining whipped cream over the ganache and spread into an even layer. Cover and refrigerate for at least 4 hours or up to overnight.

When ready to serve, use a vegetable peeler to shave 2 ounces semisweet chocolate into thin shards if desired. Sprinkle over the top. Grasping the excess parchment, remove the slab from the pan and place on a cutting board. Cut into 10 pieces, wiping the knife between each cut for cleaner-looking slices.