In a medium bowl, whisk together the flour, baking powder, and salt. In a separate large bowl, cream together the softened butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla extract. Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined. Be careful not to overmix.
Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan. Pour the batter into the prepared pan and spread evenly. Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes, then invert it onto a wire rack to cool completely. Once cooled, you can trim the top to create a flat surface, if needed.
If using frozen passion fruit puree, thaw it completely. Strain the puree through a fine-mesh sieve to remove the seeds (optional, but recommended for a smoother mousse). You should have about 1 cup of passion fruit juice.
In a medium saucepan, combine the passion fruit puree, sugar, and egg yolks. Whisk constantly over medium heat until the mixture thickens slightly and coats the back of a spoon (about 5-7 minutes). Be careful not to let it boil or the egg yolks will scramble. Remove from heat and set aside.
In a small bowl, sprinkle the gelatin over the cold water. Let it stand for 5 minutes to soften.
Add the bloomed gelatin to the warm passion fruit mixture and stir until completely dissolved. Let the mixture cool slightly.
In a large bowl, whip the heavy cream until stiff peaks form.
Gently fold the whipped cream into the cooled passion fruit mixture in two additions, being careful not to deflate the cream. Mix until just combined.
Place the cooled cake on a serving plate or cake stand. Pour the passion fruit mousse over the cake, spreading it evenly.
Cover the cake with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, to allow the mousse to set completely.
In a small saucepan, whisk together the passion fruit puree, sugar, and cornstarch. Cook over medium heat, stirring constantly, until the glaze thickens and becomes clear (about 2-3 minutes). Remove from heat and let it cool slightly.
Pour the cooled glaze over the chilled cake, spreading it evenly. Return the cake to the refrigerator for another 30 minutes to allow the glaze to set.