Preheat oven to 400 degrees F.
Melt the 3 tablespoons butter in a small bowl. Using a pastry brush coat aluminum molds (they work best) with the butter and dust the molds with ¼ cup of Dutch cocoa powder. Set tins aside on rimmed baking pan.
Melt the dark chocolate and butter together in a bowl over steaming water, stirring until smooth.
With stand mixer, whisk the eggs and sugar together to ribbon stage. Remove bowl from mixer and fold small amount of chocolate into egg mixer to temper the eggs (keep them from curdling or cooking). Fold in remaining chocolate. Sift flour, salt, and remaining Dutch cocoa powder on top of batter and gently fold mixture until combined. Place the batter into a sealable plastic bag. Batter can be made in advance and refrigerated for 2 days, but bring batter to room temperature before baking.
Cut the corner of the batter-filled bag and fill the molds 2⁄3 full. Bake for 8-10 minutes. Top of cakes should look cracked. Turn mold upside down but do not remove until ready to serve. Place raspberry or mint in center of each and serve immediately on individual plates or serving plater.
Optionally, serve with caramel sauce or Chantilly cream.