01 - Preheat oven to 350°F. Line an 8-inch springform pan with parchment paper, lightly greasing the bottom to help it adhere.
02 - In a medium bowl, sift together the flour, cocoa powder, baking powder, and salt. Whisk in the sugar until evenly distributed.
03 - In a separate bowl, whisk the eggs, melted butter, and vanilla extract until smooth and well blended.
04 - Pour the wet mixture into the dry ingredients and fold together gently until just combined. Do not overmix. Spread the batter evenly into the prepared springform pan.
05 - Bake for 20 to 25 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool completely in the pan on a wire rack.
06 - Soften the gelatin sheets in a bowl of cold water for about 5 minutes. Meanwhile, combine the raspberries, sugar, and water in a saucepan over medium heat. Cook until the berries break down, about 5 minutes, stirring occasionally.
07 - Puree the raspberry mixture with a blender, then press through a sieve to remove all seeds. Gently warm the strained puree and dissolve the softened gelatin into it. Let cool to room temperature, then pour over the cooled cake base. Refrigerate until set, about 1 hour.
08 - Melt the chopped dark chocolate over a double boiler or in the microwave in 30-second intervals, stirring between each. Let cool slightly until just warm to the touch.
09 - In a mixing bowl, beat the egg yolks with half of the sugar (about 2 tbsp) and the vanilla extract until pale, thick, and creamy. Fold in the melted chocolate until fully incorporated.
10 - In a spotlessly clean bowl, whip the egg whites with a pinch of salt until soft peaks form. Gradually add the remaining sugar and continue beating until stiff, glossy peaks hold their shape.
11 - In a separate chilled bowl, whip the cold heavy cream to soft peaks, being careful not to overwhip.
12 - Gently fold the whipped cream into the chocolate-yolk mixture until mostly combined. Then delicately fold in the whipped egg whites in two additions, preserving as much volume as possible. The mousse should be smooth, light, and airy.
13 - Pour the chocolate mousse over the set raspberry layer, smoothing the top with an offset spatula. Cover with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, until fully set.
14 - Release the cake from the springform pan and transfer to a serving plate. Arrange fresh raspberries on top, scatter chocolate curls, and finish with a light dusting of powdered sugar if desired. Slice with a warm knife for clean portions.