For the panettone crumbs: Preheat the oven to 300 degrees F. Tear 2-inch pieces of panettone into a rimmed baking sheet. Bake 20 minutes. Turn off the oven and leave the crumbs in for an additional 20 minutes. Cool completely. Puree in batches in a high-speed blender for 1 minute on high or until fine crumbs are formed.
For the sformato: Raise the oven temperature to 325 degrees F. Butter a 12-cup muffin tin and dust with panettone crumbs. Set aside.
Melt the butter in a medium saucepan over medium heat. Whisk in 3/4 cup panettone crumbs until smooth and fragrant, about 1 minute. Whisk in the milk, slowly, until smooth, then season with the salt. Bring the mixture to a simmer, whisking often, until it thickens, about 4 minutes. Remove from the heat.
Add the chopped chocolate and ricotta to a medium bowl and pour the warm milk mixture over the top. Allow to sit for 1 minute, then whisk to combine. Allow to cool until just warm to the touch. Whisk in the eggs until smooth, then divide among the prepared muffin cups. Place on a rimmed baking sheet and place in the oven. Use a kettle or pitcher to pour warm water into the baking sheet coming halfway up the sides of the muffin tin. Bake until just set, about 45 minutes. Allow to cool slightly in the pan, about 20 minutes, then invert onto a platter to cool for 1 hour. Refrigerate to cool completely. Serve topped with sweetened whipped cream and a dusting of more panettone crumbs.