1. Brew the coffee or espresso and let it cool to room temperature. Stir in 1/4 cup of the eggnog and the dark rum or bourbon, if using, to create a flavorful soaking mixture for the ladyfingers.

2. In a medium mixing bowl, beat the mascarpone cheese until smooth. Add the remaining eggnog, powdered sugar, and vanilla extract, and mix until the mixture is creamy with no lumps.

3. In a separate chilled bowl, whip the heavy cream to soft peaks. Gently fold the whipped cream into the eggnog mascarpone mixture until fully combined and light, taking care not to deflate the cream.

4. Quickly dip each ladyfinger into the cooled coffee mixture, turning just to coat the outside without letting the cookies become soggy. Break them as needed to fit the bottom of 6 small glass dessert cups.

5. Spoon a layer of eggnog cream over the soaked ladyfingers in each cup, smoothing the top. Dust lightly with cocoa powder and a pinch of ground nutmeg.

6. Repeat with another layer of dipped ladyfingers and more eggnog cream, finishing with a smooth top layer of cream that reaches just below the rim of each cup.

7. Cover the cups with plastic wrap and refrigerate for at least 4 hours, or up to overnight, until the layers are set and the ladyfingers have softened into a cake-like texture.

8. Just before serving, top each cup with a swirl of whipped cream or whipped topping if desired, then dust with additional cocoa powder and ground nutmeg. Serve well chilled.