Preheat oven (convection if possible) to 325 degrees F. In a heatproof bowl fitted over a pot of simmering water, melt the butter and chocolate, stirring occasionally. Meanwhile, prepare the ramekins with nonstick spray. Set aside.
In a stand mixer, beat the egg whites with 1 tablespoon sugar until soft peaks begin to form. Meanwhile, add cocoa powder to the melted chocolate-butter mixture and stir to combine. Add the egg yolks and stir again to combine. Set aside. When the egg whites are silky, with soft peaks, slowly add the remaining sugar. Then, using a spatula, gently incorporate the whites into the chocolate mixture. Hold the bowl with one hand and carefully spin it with the other, folding the batter the entire time.
Evenly divide the batter into ramekins, place on a baking sheet (for easy handling and even baking), and bake 15 minutes.
Cakes are done when a tester inserted into the center of the cake comes out clean. Cool several minutes, then serve in the ramekins, garnished with powdered sugar and raspberries. Or un-mold by placing the ramekins directly over a stovetop flame for several seconds to loosen, then invert them onto a serving plate and garnish.