Step 1: Crush the biscuits into fine crumbs and mix with melted butter. Press the mixture firmly into the bottom of a 9-inch springform pan and refrigerate for 20 minutes.
Step 2: In a small bowl, sprinkle the gelatin over cold water and let it sit for 5-10 minutes until it blooms.
Step 3: Puree the mango flesh with sugar and lemon juice until completely smooth. Strain through a fine-mesh sieve into a large bowl.
Step 4: Gently heat the bloomed gelatin in the microwave for 15-20 seconds until fully melted. Stir a few tablespoons of mango puree into the gelatin to temper it, then stir the gelatin mixture back into the main mango puree.
Step 5: In a separate chilled bowl, whip the heavy cream and vanilla extract until soft peaks form.
Step 6: Gently fold the whipped cream into the mango mixture in three additions, using a spatula to keep the mousse light and airy.
Step 7: Pour the mousse over the prepared base. Smooth the top with an offset spatula and refrigerate for at least 6 hours, or preferably overnight, until set.