Spray a standard 12 cup muffin pan with nonstick cooking spray. line the cups with paper liners and spray liner with cooking spray. Dust each mold with cocoa powder, tapping out the excess.
Combine the butter and cream in small saucepan over medium-high heat. Thinly shave 5 ounces of the chocolate with a large knife and put the chocolate into a medium heatproof bowl. When the cream mixture comes to a simmer, pour it over the chocolate and mix gently to incorporate the ingredients.
In a medium bowl, whisk together the eggs, sugar, mayonnaise, cornstarch, and cinnamon just until the sugar has dissolved. Pour the chocolate mixture into the egg mixture and gently mix just until the homogeneous. Mixing too much will prevent the eggs from rising in the oven. Scoop about 1/4 cup of the batter into each mold (about 3/4 of the way up the sides). Bake just until a toothpick inserted in the center comes out clean, 40 to 45 minutes. While hot, add a chunk of remaining 3 ounces of chocolate in the center of each, by gently pushing it through the top of the cupcake in the center. Cool the pan on a rack for 10 minutes and then unmold them. Serve warm on a pedastal or platter, stacked two-high if possible in pyramid form. Dust with powdered sugar and serve.