Make the streusel

1. Break the gingerbread cookies into coarse crumbs using a food processor or by placing them in a bag and crushing with a rolling pin.
2. Transfer the crumbs to a bowl and stir in the melted butter until the mixture clumps together slightly.
3. Spread the mixture in a thin, even layer on a parchment-lined baking sheet and bake at 350°F (175°C) for about 5 minutes to set and crisp.
4. Let it cool completely, then portion into the bottoms of serving jars or bowls.

Prepare the mousse

1. Put the mixing bowl and whisk (or whisk attachment) in the fridge so everything is cold before whipping the cream.
2. Sprinkle the gelatin over 2 Tbsp of cold water and let it sit to bloom while you work on the custard.
3. Combine the milk and eggnog in a measuring jug. Pour out 1/2 cup of that mixture into a separate bowl and set aside.
4. Heat the remaining 1 1/2 cups of the milk/eggnog with the vanilla, nutmeg, cloves and cinnamon in a saucepan over medium heat until it comes to a gentle boil, stirring occasionally.
5. In a separate bowl whisk together the reserved 1/2 cup milk/eggnog, egg yolks, sugar, and cornstarch until smooth and pale.
6. Temper the yolk mixture by slowly adding a thin stream of the hot milk mixture while whisking constantly.
7. Return the combined mixture to the saucepan and cook over medium-high heat, whisking continuously, until it thickens and reaches a boil. Remove from the heat.
8. Stir the bloomed gelatin into the hot custard until fully dissolved.
9. Pour the custard through a fine-mesh sieve into a clean bowl to remove any bits, then press plastic wrap directly onto the surface to prevent a skin from forming. Cool to room temperature (use an ice bath to speed this up if desired).
10. Whip the chilled heavy cream to stiff peaks. Gently fold the cooled custard into the whipped cream in two additions, keeping the mixture light and airy.
11. Pipe or spoon the finished mousse over the gingerbread streusel in your jars or bowls. Refrigerate for 1–2 hours (or longer) to set before serving.