Preheat the oven to 300° F.
Put the cupcake liners in the greased pan, and preheat the pan, too.
Combine the butter and cream in small saucepan over medium-high heat.
Chop 5 ounces of the chocolate and put it in a medium bowl.
Cut the remaining 3 ounces into ¼ ounce chunks and reserve.
When the cream mixture comes to a simmer, pour it over the 5 oz of chocolate and mix gently to incorporate the ingredients. Let cool. (This is a ganache).
In a medium bowl, whisk together the eggs, sugar, oil, cocoa powder and cornstarch until the sugar has dissolved.
Pour the cooled ganache into the egg mixture and gently mix just until the batter is smooth.
Scoop about ¼ cup of the batter into each mold (about ¾ of the way up the sides).
Tuck a ¼ ounce chunk of the reserved chocolate into the center of each cupcake.
Bake until the edges are set and the center still looks a little moist, about 30 minutes.
Cool the pan on a rack for 10 minutes and then unmold the budinos.
Dust them with powdered sugar, and serve warm.