Separate rosemary into small bunches. Beat egg white with a drop or two of water until it is very loose. Place superfine sugar in a small, shallow bowl. Lightly brush one sprig of rosemary with the egg white. Sprinkle superfine sugar over the damp bunch of rosemary; shake to remove excess. Transfer rosemary to a cooling rack to dry, at least 1 hour. Repeat until all rosemary is used; set aside sugared rosemary. (It will keep at room temperature for about 24 hours.)
Preheat the oven to 275 degrees with two racks centered. In the top of double boiler, or in a heat-proof bowl set over simmering water, melt butter and chocolate together, stirring frequently, about 5 minutes. Set mixture aside to cool.
In a medium bowl, lightly beat egg yolks, and stir them into the cooled chocolate mixture. Add granulated sugar, flour, and baking powder to the chocolate mixture, and stir to combine.
In the bowl of an electric mixer fitted with the whisk attachment, whisk the egg whites just until soft peaks form. Stir one-third of the egg whites into the chocolate mixture to lighten it. Fold the chocolate mixture into the remaining egg whites, just until combined. Spoon the batter into twelve 3 3/4-inch heart molds with removable bottoms. Transfer molds to a rimmed baking sheet. Bake until centers spring back when touched, about 45 minutes. Transfer molds to a wire rack, and let sit 5 minutes. Unmold cakes. (After they have cooled, if not using right away, cover cakes lightly with plastic wrap.)
To serve, place a cake on serving plate. Pour warm ganache over it; sprinkle with praline, if using. Top with a second and then a third cake layer, drizzling each with ganache. Repeat to make 3 more servings. Sprinkle cakes with praline if desired, and garnish with sugared rosemary. Serve.