Line a 9 inch pie plate with plastic wrap, leaving a few inches overhang on all edges.
Whisk the eggs with the sugar in a large bowl, adding the cornstarch whisking until no lumps remain.
In a medium saucepan, add the cream and milk and simmer over low heat for a couple of minutes. Turn off the heat and slowly pour about 1 cup of the hot milk-cream into the egg bowl, whisking steadily, to temper the eggs. When blended, slowly pour the tempered eggs back into the saucepan of milk and cream, again whisking constantly.
Put the pan back over medium-low heat, and whisk steadily until the custard heats and thickens. Don’t let it boil, though it should bubble occasionally. Cook for 7 to 8 minutes, until it is quite thick.
Remove from the heat, and stir in the crumbled biscotti, chopped chocolate, nuts, and cocoa powder. Stir continuously until the chocolate has melted and all the ingredients are evenly blended. Pour the custard into the pie plate, and cover with plastic wrap, laying it directly on the top of the pudding. Refrigerate until set, at least 4 hours or overnight.
To serve: Remove the top sheet of plastic, invert onto a serving plate, and cut into wedges. Top with unsweetened whipped cream and coarsely crumbled biscotti (if you have any left) or crushed hazelnuts; in season, strawberries or raspberries would be wonderful.