In a small saucepan, melt butter over medium-low heat and stir in graham cracker crumbs, chopped pecans, brown sugar, and sea salt. Toast for exactly 2 minutes, stirring constantly until fragrant and golden. Transfer to a small bowl and cool completely—this becomes your crumble topping.

In a separate medium bowl, whisk together pumpkin puree, mascarpone cheese, vanilla extract, cinnamon, nutmeg, ginger, and cloves until completely smooth. No lumps should remain in the pumpkin mixture; use the back of a spoon to crush any stubborn mascarpone bits against the bowl's side.

In a chilled mixing bowl, pour the heavy whipping cream and add granulated sugar. Using an electric mixer on medium-high speed, whip for 3–4 minutes until soft peaks form—the cream should hold its shape but still look slightly pillowy, not stiff.

Gently fold the pumpkin mixture into the whipped cream using a rubber spatula in three additions. Work slowly and deliberately to maintain the mousse's airiness; overmixing deflates the delicate structure and results in dense, heavy texture instead of the desired ethereal consistency.

This creamy pumpkin mousse comes together when you fold in maple syrup during the final combination step, stirring just until no white streaks remain. Divide the mousse evenly among six serving glasses or bowls, filling each about three-quarters full to leave room for toppings.

Top each serving with a generous spoonful of the cooled graham cracker crumble, pressing gently so it adheres to the mousse's surface. Serve immediately or refrigerate up to 4 hours before serving—the recipe tastes best when served chilled.