Adjust an oven rack to the lower-middle position and preheat oven to 325 degrees. Grease a mini cheesecake pan liberally with cooking spray.

In a stand mixer using the whisk attachment, beat the eggs at high speed until the volume doubles (about 5 minutes).

Meanwhile, melt the chocolate and butter together. You can either do this on the stove (by placing the chocolate and butter in a large heatproof bowl set over a pan of almost-simmering water until smooth and very warm – about 115 degrees on an instant-read thermometer – stirring occasionally). Or you can do this in the microwave (by heating the chocolate in a microwave-safe bowl at 50% power for 2 minutes, then stirring, the adding the butter, and continuing to heat at 50% power and stir in 1-minute intervals until the mixture is well-melted and smooth). Then fold about a third of the beaten eggs into the chocolate mixture using a large rubber spatula until only a few streaks of egg are visible. Fold in half of the remaining egg foam, and then the last half of the foam, until the mixture is streak-free and smooth.

Scrape the batter into the mini cheesecake pans, filling each cup about 2/3 full. (I used about 5 tablespoons of batter per cake.) Bake until the cakes have risen slightly, the edges are just beginning to set, a thin-glazed crust (like a brownie) has formed on the surface, and an instant-read thermometer inserted halfway into the center reads 140° F, 14-18 minutes. Remove the pan and set on a wire rack; cool to room temperature. Cover and refrigerate overnight to mellow and firm. (The cakse can be refrigerated for up to 4 days.)

To serve, carefully remove the mini cheesecakes from their cups, and top with whipped cream. Garnish with sprinkles if desired.