Preheat the oven to 375 degrees F. Grease four 10-ounce ramekins with butter and arrange on a baking sheet.
In a stand mixer, whip together the eggs, yolks and sugar until the mixture is light in color, doubled in volume and falls in ribbons. Set aside.
In a double boiler set over medium heat, melt the bittersweet chocolate, semisweet chocolate, butter and espresso powder. Stir until smooth. Fold the melted chocolate into the eggs.
Sift together the flour and salt. Sprinkle the dry mixture over the chocolate-egg mixture and gently fold in.
Divide the batter between the greased ramekins. Bake for 20 minutes, or until the top is set and the edges are firm but there is a slight wiggle in the center.
Run a knife around the edge of the ramekins and invert the cakes onto plates. Dust with confectioners' sugar. Garnish with raspberries if using. Serve warm.