Preheat the oven to 350, liberally butter the bottom of a 10-side springform pan or spray with nonstick cooking spray and set aside.
For the torte:
Sift the flour, baking powder and salt together twice and set aside.
Cream the butter, sugar, and vanilla with an electric mixer at high speed for 5 minutes, till light and fluffy.
Reduce the mixer speed to low and beat in the eggs, one by one.
Beat in the melted chocolate and if using, the hazelnuts.
Add the sifted dry ingredients alternately with the milk, beginning and ending with the dry and beating by hand after each addition only enough to incorporate.
Spoon the batter into the prepared pan and smooth the top.
Rap the pan several times on the counter to release any large air bubbles, then bake about 1 hour and 20 minutes, till the torte begins to pull from the sides of the pan, is springy to the touch, and a cake tester inserted in the middle comes out clean.
Cool the torte in the upright pan on a wire rack for 15 minutes, then loosen it around the edge with a small spatula.
Release and remove the springform pan sides and cool the torte, still on the pan bottom and cake rack, to room temperature.
For the eggnog:
Blend the sugar-flour mixture with the milk in a small heavy saucepan.
Set over moderate heat and cook, stirring constantly, about 3 minutes, till thickened and smooth.
Whisk a little of the hot mixture into the beaten yolk, stir back into the pan, and cook and stir over moderately low heat for 2 minutes longer.
Remove from heat, mix in the rum extract, then pour the eggnog through a fine sieve into a metal bowl.
Set in an ice bath and chill 15 - 20 minutes, whisking often to prevent a skin from forming on the top.
Pour the eggnog into a plastic squirt bottle and refrigerate till ready to use. (I would think you could also use a plastic baggie with a small hole cut in one corner if you don't have a squirt bottle.).
To frost the torte:
set the torte, still on the springform pan bottom on a lazy susan and frost the top and sides smoothly and thinly with the whipped cream.
Starting in the center of the torte and exerting steady but gentle pressure on the squirt bottle, apply the eggnog in a tight spiral on the top of the torte, spacing the lines about 1/2 inch apart. (I'm sure you can do this without a lazy susan - you'll just have to move the bottle instead of the torte).
Scatter the chocolate sprinkles over the top of the torte, if you like, then transfer, still on the springform pan bottom, to a cake plate and serve.