For the Brownie Base:

1. Preheat your oven to 350°F (175°C). Grease and line an 8×8-inch baking pan with parchment paper.
2. In a medium bowl, whisk together the melted butter and sugar until smooth.
3. Beat in the eggs one at a time, then stir in the vanilla extract.
4. In a separate bowl, mix the flour, cocoa powder, baking powder, and salt.
5. Gradually add the dry ingredients to the wet mixture, stirring until just combined.
6. Pour the batter into the prepared pan and bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean. Let the brownies cool completely.
7. Once cooled, cut out heart shapes using a cookie cutter (or slice into squares if you prefer).

For the Raspberry Mousse:

1. In a small saucepan, cook the raspberries, sugar, and lemon juice over medium heat for about 5 minutes, stirring occasionally, until the raspberries break down into a puree. Strain the mixture through a fine mesh sieve to remove the seeds, then set the raspberry puree aside to cool.
2. In a large mixing bowl, whip the heavy cream, powdered sugar, and vanilla extract until soft peaks form.
3. Stir the dissolved gelatin into the raspberry puree, and then gently fold the raspberry mixture into the whipped cream until fully combined.
4. Spoon or pipe the mousse over the cooled brownie hearts, filling each heart-shaped mold or pan.
5. Refrigerate for at least 2-3 hours or until the mousse is set.

To Serve:

1. Carefully remove the mousse from the molds or pan.
2. Garnish with fresh raspberries, mint leaves, and a sprinkle of chocolate shavings if desired.