In a small bowl, sprinkle the gelatin over the cold water. Let it sit for 5-10 minutes to bloom.

In a medium saucepan, combine milk, coffee, brown sugar, molasses, and spices. Heat over medium heat until steaming (do not boil). Remove from heat and whisk in the bloomed gelatin until dissolved.

In a separate bowl, whisk the egg yolks. Slowly drizzle in half of the hot milk mixture while whisking constantly. Pour the tempered egg mixture back into the saucepan.

Return the saucepan to low heat and cook, stirring constantly, until the mixture thickens enough to coat the back of a spoon (3-5 minutes). Do not let it boil. Strain through a fine-mesh sieve into a large bowl and let it cool completely to room temperature.

In a separate chilled bowl, whip the cold heavy cream until stiff peaks form.

Gently fold the whipped cream into the cooled gingerbread base in two additions until just combined. Be careful not to overmix. Divide the mousse among 6 serving glasses, cover, and refrigerate for at least 4 hours, or until set.

Before serving, top with additional sweetened whipped cream and crushed gingersnap cookies, if desired.